Plugin/extension documentation is very disorganised, out of date, in many locations

If you're looking to write a plugin/extension, currently it is a real mess to find information:

• The wiki has some half written pages under the heading of "Extensions" ( manifest), including some incomplete and presumably out of date manually written API documentation
• Under the heading of "Legacy plugin API", the wiki links to:
  • Updating plugins for MuseScore Studio 4.4 (wiki page)
  • https://musescore.org/en/node/337468#Tips-for-adapting-plugins-for-4.x (apparently the newest documentation on musescore.org, but it's just a kind of collection of tips for updating 3.x plugin to 4.x format)
  • "Legacy API documentation" (Doxygen generate API docs for the 3.5 API)
• Meanwhile there is a much more complete (but old, but apparently still updated?) page about the 3.x plugin API in the handbook: https://musescore.org/en/handbook/developers-handbook/plugins-3x
• And finally it turns out that the actual current plugin API documentation is on musescore.github.io. I didn't find this information anywhere, someone told me on Discord.

There is also the separate issue that it seems at some point the term "plugin" was ditched for "extension", except that the UI continues to happily use the term Plugin, which is super confusing.

I know it would be a big project to fix all of this, but maybe a start would be to:

• remove the wiki pages that are very incomplete and not really adding anything
• prominently link to the current API docs in many places (including at the top of the 4.x and 3.x plugin pages in the Handbook)
• reorganise and provide at least a basic high level view of the current state of play in the wiki.

(Related, but not really overlapping: #25703)