ci: bundle Mkdocs site into Releases

[jamietanna]

Changes

As we now perform the Mkdocs build as part of the main repo, we can
now attach the docs site build into the GitHub Release artifacts as part
of our release pipeline.

For folks who are running versions behind the "bleeding edge" of
Renovate, this is very useful, as they can now grab a copy of the built
documentation for their given Renovate version.

This allows folks to take the mkdocs-site.tgz and upload it to static
hosting, without restoring to workarounds0 that can make it more
difficult.

Because we're now building the docs site as part of the release, we need
to upload the built artifact to then be used by our deploy step.

As the deploy step no longer needs to build the docs site, we can
simplify this.

However, if the release hasn't occurred (because there are no commit(s)
that need a release right now) we still need to rebuild the docs site,
in case there are docs-only changes.

Context

Please select one of the below:

• This closes an existing Issue: Closes #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe): Claude Sonnet 4.5 (GitHub Copilot + crush)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository:

[jamietanna]

OK, let's give this a go - if it doesn't work we can revert

Worst case, we'll try a couple of attempts, and revert back before EOD today

[jamietanna]

It's run in the case that no release is made (https://github.com/renovatebot/renovate/actions/runs/22231825236/job/64313402746) - I'll keep an eye on when a release is made