docs(android): add Java API Reference (Javadoc) link, sidebar entry, …

[madhesh60]

Summary

Fixes #19356 — Unify and Improve the Android dev story

Changes

• Added Java API Reference (Javadoc) link to using-executorch-android page (1-line fix)
• Added Javadoc entry to Android sidebar navigation
• Created package-info.java for org.pytorch.executorch with overview and quick-start
• Created package-info.java for org.pytorch.executorch.extension.llm
• Created overview.html with top-level intro, quick-start code, and package descriptions
• Updated build.gradle to pass -overview flag so overview.html is picked up by the build

Testing

All are verified by opening the documents manually.

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

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

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

cc @mergennachin @AlannaBurke

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19508

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⚠️ 11 Awaiting Approval

As of commit 661de6f with merge base af90130 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Cadence Build & Test (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test Vulkan Backend (gh)
• Test XNNPACK Backend (gh)

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[madhesh60]

@pytorchbot label "release notes: none"

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