docs: version selectors synchronization

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain:

Fixes #18229

Documentation & Build related.

What changes did you make? (Give an overview)

• Support vNN.x banches
• Fetch versions from the main branch on GitHub
• Update data on each release
• Trigger rebuilds of docs sites.

Is there anything you'd like reviewers to focus on?

Changes in docs/ should be ported to the v8.x branch. Then, the latest branch should be updated.

Question: What shall we do with eslint.org/docs/next/ when we release v9 final? Keep it or remove it?

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[nzakas]

Question: What shall we do with eslint.org/docs/next/ when we release v9 final? Keep it or remove it?

I think we should redirect it to /docs/latest.

[nzakas]

LGTM and a lot more straightforward than what we've been doing.

Just leaving open a couple days to give others a chance to review.