Make Addons/WebExtensions docs less Mozilla-biased

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