Populate "Using ExecuTorch on Android" doc page

[GregoryComer]

📚 The doc issue

Note: This issue pertains to the executorch-just-works branch. It is intended to merge back into master within the next 1-2 weeks.

Populate the top-level Android docs under using-executorch-android.md. Top-level platform docs for runtime integration on Android. Focus on Java APIs and acquiring the ET libraries though Maven/Gradle. Cover library installation and runtime APIs for loading + running a model using Java runtime APIs. Briefly cover relevant delegates (XNN + Vulkan + NPU).

Placeholders and skeleton is added in #8511.

General documentation guidelines that I'm hoping to follow:

• Leverage brief in-line code samples, where possible
• Keep things concise
• Avoid first person (I/we)
• I'm generally trying to follow https://developers.google.com/style/highlights

Suggest a potential alternative/fix

Updates

• [Android] Rename build script #8747
• [Android] Add a list of artifact history #8797
• [Android docs] Minor update to existing sections #8802
• [Android docs] Add building instructions and code snippet #8811

No response

cc @mergennachin @byjlw @kirklandsign @cbilgin

[mergennachin]

This is a good issue to make sure user's issue is resolved in the doc

#8636

[kirklandsign]

Per suggestion from discord, we need to note down how to load a model, what model can be loaded, how to infer during run-time, what input format should be, etc.

[mergennachin]

Also #8665 is a relevant issue

[kirklandsign]

Do we still need https://github.com/pytorch/executorch/blob/main/docs/source/getting-started.md?plain=1#L103-L140 given we have https://pytorch.org/executorch/main/using-executorch-android.html?