Add automated doc flow for release

[mthrok]

Currently, nightly documentation is pushed automatically for each commit and on daily basis.

For release, automating it is tricky because we need to ensure that the version number is properly updated.
(removing alpha suffix at branch cut time and updating minor version at minor release)

For this reason, in release/2.1 we used manual-trigger to build and deploy doc for release.
This commit back port it to main.

For each release, once the version number is updated, running build_doc job from GHA will deploy the documentation automatically.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/audio/3634

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures, 2 Unrelated Failures

As of commit 4b07775 with merge base d9942ba ():

NEW FAILURES - The following jobs have failed:

• Build MacOS Wheels / pytorch/audio / wheel-py3_8-cpu (gh)
• Build Windows Conda / pytorch/audio / conda-py3_8-cuda11_8 (gh)
• Unit-tests on Linux CPU / tests (3.9) / linux-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Build M1 Conda / pytorch/audio / conda-py3_8-cpu (gh)
• Build MacOS Conda / pytorch/audio / conda-py3_8-cpu (gh)

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