docs: fix old info in contributing guide (#5779)

**What's the problem this PR addresses?**
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EDIT: I just saw that #5776 beat me to it. However, the version I
propose adds clarity imho as well as links to relevant resources.

---

The instructions in the 'Writing documentation' section gave false
information (I assume it was simply outdated), making it harder for
people to contribute.

Pls tag for Hacktoberfest.
...

**How did you fix it?**
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* Revised the 'Writing documentation' section so that it aligns with our
current project structure
* Fixed broken links
* Provided links to the tech we use  
...

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- [x] I have read the [Contributing
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Co-authored-by: Maël Nison <[email protected]>

packages/docusaurus/docs/advanced/04-technical/contributing.md

@@ -125,18 +125,20 @@ It's generally seen as [bad form](https://twitter.com/brian_d_vaughn/status/1224
 
 ## Writing documentation
 
-Our website is stored within the [`packages/docusaurus`](https://github.com/yarnpkg/berry/tree/master/packages/docusaurus) directory. *Do not manually edit the html files in the `docs` folder!* Instead, just make your changes in the Gatsby directory (for example you'd edit this very page [here](https://github.com/yarnpkg/berry/blob/master/packages/docusaurus/docs/advanced/04-technical/contributing.md)), then run the following command to spawn a local server and see your changes:
+We use [Docusaurus](https://docusaurus.io/docs) to generate HTML pages from [mdx](https://mdxjs.com/docs/) sources files. 
+
+Our website is stored within the [`packages/docusaurus`](https://github.com/yarnpkg/berry/tree/master/packages/docusaurus) directory. You can change a page by modifying the corresponding `.mdx` file in the `docs` folder. For example, you'd edit this very page [here](https://github.com/yarnpkg/berry/blob/master/packages/docusaurus/docs/advanced/04-technical/contributing.md).
+
+Then run the following command to spawn a local server and see your changes:
 
 ```bash
 yarn start
 ```
 
-Once you're happy with what the documentation looks like, just commit your local changes and open a PR. Netlify will pick up your changes and spawn a fresh preview for everyone to see:
+Once you're happy with what the documentation looks like, just commit your local changes and open a PR. Netlify will pick up your changes and create a fresh preview for everyone to see:
 
 ![](https://user-images.githubusercontent.com/1037931/61949789-3cc09300-afac-11e9-9817-89e97771a4e1.png)
 
-Once everything is green and a maintainer has reviewed your changes, we'll merge them and a bot will automatically trigger a rebuild of the website and update the `docs` folder 🙂
-
 ## Profiling
 
 Run the following command to generate an unminified bundle: