Unify and Improve the Android dev story

[psiddh]

📚 The doc issue

Add a "Java API Reference" hyperlink directly in the using-executorch-android page → trivial 1-line fix

Add sidebar/nav entry under Platforms → Android → "Java API Reference (Javadoc)"

Improve the bare javadoc/index.html with an overview, quick-start code, and package descriptions

Unify the Android dev story — today it's fragmented across the usage guide, Javadoc, demo apps, and the GitHub README

Suggest a potential alternative/fix

No response

cc @mergennachin @AlannaBurke

[madhesh60]

/assigntome