Update build from source and getting started docs

[GregoryComer]

Summary

Update Getting Started and Build from Source Docs:

• Integrate Windows steps into the main flow with minor Windows-specific callouts.
• Clarify top-level flow for building from source - add a table by use case.
• Clarify building ET as a submodule vs standalone build.
• Re-order, re-word, and clean up the content related to building from source.
• Add info on NDK build for Android.

Tracked in #14791 and #14759.

cc @mergennachin @byjlw

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15311

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures, 55 Pending

As of commit 8d9b821 with merge base fe3c1dc ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b22ccfab33d45408c2ee04ca1549f67be83c7542f86631532f2faaf7b57fec2e /exec failed with exit code 1
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

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[mergennachin]

I see that you removed but I recommend adding back a brief "Verify Your Build" section after the Building section that contains

• One-line command to export a simple model
• One-line command to run it with the executor_runner

[GregoryComer]

I see that you removed but I recommend adding back a brief "Verify Your Build" section after the Building section that contains

• One-line command to export a simple model
• One-line command to run it with the executor_runner

Sounds good. I'll re-add. The reason I removed it was partly because we don't actually build the executor_runner for most presets and it requires multiple CLI options to include. I'm a little conflicted on this, but maybe we should build by default?

I think it does make sense to have a quick verification step. I'll re-add.

[GregoryComer]

I see that you removed but I recommend adding back a brief "Verify Your Build" section after the Building section that contains

• One-line command to export a simple model
• One-line command to run it with the executor_runner

Sounds good. I'll re-add. The reason I removed it was partly because we don't actually build the executor_runner for most presets and it requires multiple CLI options to include. I'm a little conflicted on this, but maybe we should build by default?

I think it does make sense to have a quick verification step. I'll re-add.

Added.