Document QT_ANDROID_ABIS and its friends

Document the available ABI values on each documentation page with something like
"These are the common values: a,b,c"

Add overview documentation for how all of these ANDROID vars and properties interact. Use blog post as inspiration.

Semi-related, mention in documentation that qt_add_library + Android for an SDK use case is currently not supported.

Mention in docs that qt_add_application + qt_add_library linked into the application works

Investigate and see if it's possible to implement a diagnostic: if an app is built with 2 abis, but a library it links to with only one abi, we should show an error at CMake time, rather than androiddeployqt time.