Update build from source docs

[GregoryComer]

Summary

Overhaul the "Building from Source" doc page. The primarily intent of these changes is to document CMake presets and the various build options that we expose. However, I also did a pass on the existing contents of the file to improve formatting and clarity.

I've re-organized the page to clearly delineate environment setup, python install, and native build. It should flow better and be easier to read.

Test plan

I have built the docs locally to inspect the contents for formatting and correctness.

Preview page: https://docs-preview.pytorch.org/pytorch/executorch/13210/using-executorch-building-from-source.html
Live page (for comparison): https://docs.pytorch.org/executorch/0.7/using-executorch-building-from-source.html

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13210

• 📄 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, 1 Unrelated Failure

As of commit a57b89f with merge base d118a63 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (profiling) / build (gh)
• pull / test-llama-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ae0802004ff117f0d803f6d60fbfa1ba19214bd65be8c729304f5391b5722db8 /exec failed with exit code 127

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-llava-runner-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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[GregoryComer]

Closes #13196.