[SITE-1134] Add documentation for configuring a WordPress multisite o…

…n WP Composer Managed (#9173)

* add a multisite config page
copy pasta mostly

* update docs header

* add first pass of guide
need to add the images in

* add the images

* remove link to the configuration
it's wrong, and it doesn't add anything since we copy the config in the doc

* remove early access alert
it's already noted in the intro, no need to reiterate here, especially since all the subpages don't have it

* replace subdomain to subdirectory conversion with section on wpms search-replace
wpms search-replace does not work with composer-based sites yet, so for now, the workaround is still to use terminus/wp-cli

* updates from review

* rephrase intro paragraph

source/content/guides/wordpress-composer/01-introduction.md

@@ -15,7 +15,7 @@ showtoc: true
 permalink: docs/guides/wordpress-composer
 ---
 
-[Composer](https://getcomposer.org/) is a widely-used PHP dependency and package manager that provides a simplified method to manage packages (plugins, themes and -- with the [WordPress (Composer Managed) upstream](/wordpress-composer-managed), WordPress core itself) used by a WordPress site.
+[Composer](https://getcomposer.org/) is a widely-used dependency and package manager for PHP. Composer provides an easy way to manage packages used by a WordPress site. Packages can be plugins, themes, other PHP libraries and even WordPress core itself.
 
 At the most basic level, Composer requires:
 

source/content/guides/wordpress-composer/04-multisite-configuration.md

@@ -0,0 +1,72 @@
+---
+title: WordPress with Composer on Pantheon
+subtitle: Multisite Configuration
+description: Learn how to configure a WordPress multisite with Composer on Pantheon.
+contenttype: [guide]
+innav: [false]
+categories: [dependencies]
+cms: [wordpress]
+audience: [development]
+product: [--]
+integration: [--]
+tags: [wordpress]
+contributors: [jazzsequence]
+layout: guide
+showtoc: true
+permalink: docs/guides/wordpress-composer/multisite-configuration
+anchorid: multisite-configuration
+---
+
+This section provides information on how to configure your WordPress (Composer Managed) site on Pantheon for WordPress multisite. This guide assumes that you have already set up a [WordPress Multisite upstream](/guides/multisite/#request-a-wordpress-multisite) and created a new site using that upstream. If you haven't already, you should also read through our [guide on WordPress Multisite](/guides/multisite) before beginning this process.
+
+Because Bedrock uses `Config::define()` and `config/application.php` instead of traditional `define()` statements in `wp-config.php`, you will not be able to use WP-CLI via Terminus to enable multisite as documented in our standard [multisite configuration guide](/guides/multisite/config/).
+
+<Alert title="Note" type="info">
+
+Adjust placeholders in code snippets as needed throughout this guide. This includes placeholders such as `<site>` and `<env>` in Terminus commands, in addition to placeholders in brackets `<>` in larger code blocks.
+
+</Alert>
+
+## Set Up WordPress Multisite
+
+1. Set the site's connection mode to Git:
+
+    ```bash{promptUser: user}
+    terminus connection:set <site>.dev git
+    ```
+2. Open the `config/application.php` file in your site's codebase and add the following line anywhere _above_ the `Config::apply();` line:
+
+	```php
+	Config::define( 'WP_ALLOW_MULTISITE', true );
+	```
+3. Visit your development site's WordPress admin and navigate to **Tools** > **Network Setup**.
+    ![Network Setup page](../../../images/wordpress-composer/04-multisite-network-setup.png)
+4. Choose **Sub-domains** or **Sub-directories** (depending on your needs), fill in the Network Title and Admin Email, and click **Install**.
+5. You will be given instructions to update your `config/application.php` file. Below where you added the `WP_ALLOW_MULTISITE` line, copy and paste the code provided. The `SUBDOMAIN_INSTALL` value will be `true` or `false` depending on the option you chose in the previous step. For example, if you chose subdirectories, your `config/application.php` file should look like this:
+
+	```php
+	Config::define( 'MULTISITE', true );
+	Config::define( 'SUBDOMAIN_INSTALL', false );
+	// Use PANTHEON_HOSTNAME if in a Pantheon environment, otherwise use HTTP_HOST.
+	Config::define( 'DOMAIN_CURRENT_SITE', defined( 'PANTHEON_HOSTNAME' ) ? PANTHEON_HOSTNAME : $_SERVER['HTTP_HOST'] );
+	Config::define( 'PATH_CURRENT_SITE', '/' );
+	Config::define( 'SITE_ID_CURRENT_SITE', 1 );
+	Config::define( 'BLOG_ID_CURRENT_SITE', 1 );
+	```
+6. Save your changes and commit them to your Git repository. WordPress will prompt you to log back in after you push the changes.
+7. Log into your site. When you log back into your WordPress admin dashboard, you will see a new **My Sites** menu item in the top left corner. This is where you can manage your network of sites. You now have a WordPress subdirectory multisite network set up on a WordPress (Composer Managed)-based upstream.
+	![My Sites](../../../images/wordpress-composer/04-multisite-my-sites.png)
+	
+## Using Subdomain Multisite on Pantheon
+
+Pantheon does not support sub-subdomains on the `pantheonsite.io` platform domain. This means that sub-sites on a subdomain multisite will not be accessible by default. See the [Mapping Custom Hostnames](/guides/multisite/config/#map-custom-hostnames-subdomain-configurations-only) panel in the Multisite Configuration guide for how to set up a custom domain for your sub-sites.
+
+## Multisite Search and Replace with WordPress (Composer Managed)
+Currently, the built-in dashboard [WordPress Multisite Search and Replace](/guides/multisite/search-replace/) does not support Composer-based WordPress multisites. To perform a search and replace on a WordPress (Composer Managed) multisite, you will need to use WP-CLI via Terminus manually. For more information, see our [WordPress Multisite Search and Replace](guides/multisite/workflows/#run-wp-cli-search-replace-manually) guide.
+
+## More Information
+
+* [WordPress Multisite Configuration](/guides/multisite/config/)
+* [WordPress Multisite Search and Replace](/guides/multisite/search-replace/)
+* [WordPress Multisite documentation](https://developer.wordpress.org/advanced-administration/multisite/)
+* [WordPress Multisite Domain Mapping](https://developer.wordpress.org/advanced-administration/multisite/domain-mapping/)