Add AndroidX migration docs

[blasten]

This doc replaces the existing page https://flutter.dev/docs/development/packages-and-plugins/androidx-compatibility.

I added this page under development since it targets all types of Flutter projects.

[sfshaza2]

A grammatical nit to fix but, otherwise, looks good. Thanks for remembering to set up forwarding for the deleted file! LGTM, once the nit is fixed.

[sfshaza2]

Also, I see that Travis is encountering some link problems. Can you fix?

http://localhost:4002/docs/codelabs/layout-basics
- (1224:0) 'Text class' => http://localhost:4002/docs/codelabs/layout-basics#text-class (HTTP 200 but missing anchor)
http://localhost:4002/docs/development/androidx-migration
- (999:41) 'develope..' => http://localhost:4002/docs/development/developer.android.com/studio (HTTP 404)
- (999:41) 'develope..' => http://localhost:4002/docs/development/developer.android.com/studio (HTTP 404)

[sfshaza2]

LGTM!

[mklim]

LG modulo one question.

[mklim]

Have we enabled building the plugins as AARs by default? My most recent understanding is that we still have compatibility problems here, but I could be missing something.

[blasten]

yeah, it's now enabled by default. However, plugins are built as AAR only when the build failed and the heuristic we have for detecting AndroidX failure is true.

[sfshaza2]

My interpretation of this convo is that we're good to go. :)

[mklim]

Yeah, LGTM! :)