Update Addons/WebExtensions docs section on MDN

[chrisdavidmills]

We are currently in the middle of trying to move non-web platform docs off MDN, and this issue serves as a record of what is needed for webextensions. We have the add-ons landing page — https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons — but since Firefox (and other browsers) moved to WebExtensions APIs for building add-ons, the only real active part of this section (i.e not archived) is https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions.

So the work required in this case is probably along the following lines:

• Check https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons to see if there is anything on there that we want to keep apart from the WebExt bit.
• Move the https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Contact_us page to another suitable place.
• Rewrite https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions to remove any unneccessary Firefox-specific language.
• Make https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions a top-level page, i.e. https://developer.mozilla.org/en-US/docs/WebExtensions. This seems appropriate, as WebExtensions APIs are not purely a Mozilla project, nor are they really Web APIs.
• Redirect any pages that were under https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons as appropriate to avoid link breakage.
• Redirect https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons itself to https://developer.mozilla.org/en-US/docs/WebExtensions

[mixedpuppy]

There are some items in the initial comment that I have concerns over. I'd like this bug to go on hold until there is an overall plan. The largest concern (not the only) is the 3rd bullet about removing Firefox specific language. Doing that would hobble the usefulness of the documentation.