Update broken links in guidelines (mdn#17692)

* update broken links in guidelines

* Rewrite to match current practice

Co-authored-by: Jean-Yves Perrier <[email protected]>

files/en-us/mdn/guidelines/does_this_belong_on_mdn/index.md

@@ -55,22 +55,14 @@ In general, anything that isn't an open web standard does not belong on MDN. The
 
 ### Mozilla products
 
-Documentation in this category includes information on both how to work with Mozilla products, as a developer, and how to contribute to these open-source projects.
 
-While MDN Web Docs contains a large quantity of Mozilla product documentation, the focus of new content development is on open web standards. We are in the process of determining what to do about this content, so it may not make sense to create new Mozilla product documentation on MDN Web Docs. If you are working on a Mozilla product (or project that may become a product), talk to a member of the [MDN staff team](https://wiki.mozilla.org/Engagement/MDN_Durable_Team) to discuss possible avenues for documenting your product. Also, see the section on [Cases for documenting elsewhere](#cases_for_documenting_elsewhere), below.
+MDN Web Docs doesn't host documentation for Mozilla products anymore.
 
-- [Firefox browser](/en-US/docs/Mozilla/Firefox)
+In particular, the following products have their own documentation site:
 
   - [Firefox Developer Tools](https://firefox-source-docs.mozilla.org/devtools-user/index.html)
-  - [Add-ons](/en-US/docs/Mozilla/Add-ons)
-  - [Building and configuring Firefox](/en-US/docs/Mozilla/Developer_guide/Build_Instructions)
-  - etc.
-
-- [The Mozilla platform](/en-US/docs/Mozilla)
-
-  - [Gecko](/en-US/docs/Mozilla/Gecko)
+  - [Building and configuring Firefox](https://firefox-source-docs.mozilla.org/setup/configuring_build_options.html)
   - [SpiderMonkey](https://firefox-source-docs.mozilla.org/js/)
-  - etc.
 
 ### What else?