Provide Multisite requirements directly on multisite documentation

[dylanbaumann]

Closes: N/A

Summary

Multisite - Introduction - Directly shares plan requirements for WordPress Multisite on the page where customers learn about WordPress Multisites.

Effect

The following changes are already committed:

• Removes description telling people to check the /pricing page for more information about Multisite access (this page did not contain any Multisite information at the time of my writing)
• Adds clear definitions of Multisite requirements to the Multisite docs themselves, saving customers a click and getting information in their hands faster.
• Retains the link to the pricing page, if they want to learn more about the plans, but it is not required to visit.

Remaining Work and Prerequisites

• none

Dependencies and Timing

• none

Release:

• When ready

More Info:

• Pantheon Slack Thread

---

Post Launch

Do not remove - To be completed by the docs team upon merge:

• Redirect /old-path/ => /new-path/ (if applicable)
• Include/exclude pages ^ respectively within docs search service provider (if applicable)
• For Heroes - add a props post to the discussion board.
• Remove from the project board

[pantheon-decoupled]

⚡ Deployed with Pantheon Decoupled

This build was successfully deployed with Pantheon. You can track the build logs here.

👀 Preview: https://pr-8871-documentation.appa.pantheon.site
🛠️ Manage in Pantheon: https://dashboard.pantheon.io/site/2b30153f-e8b1-4427-b076-6109e704ba5d/overview