Update docs to use top-level CMake targets

[GregoryComer]

Update the "Building from Source" and "Using ExecuTorch with C++" docs to specify the top-level CMake targets. I also added some info to the selective build docs to document the CMake flags which control selective build with the executorch_kernels target.

This is done in the context of top-level CMake targets, tracked in #12293.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• -> Update docs to use top-level CMake targets #12807

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12807

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❌ 1 New Failure

As of commit e9af248 with merge base a70b7a8 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t 234b4f9eba982d5ab140697689aeab6bac92fdd27561a5095db299d9f8853385 /exec failed with exit code 1

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[mergennachin]

cc @lucylq

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[lucylq]

Can we CP this to 0.7, do these changes only reflect code in main?

[GregoryComer]

Can we CP this to 0.7, do these changes only reflect code in main?

The actual CMake changes aren't in 0.7, so this should probably wait for 1.0.