ci: build release docs from the branch tip, not the dispatch ref (#847)

Aligns the release docs deploy with apify/crawlee-python#1946, so all
three Python repos build and deploy docs the same way.

In the stable release the docs build checked out the workflow-dispatch
ref (the pre-bump commit), which predates the changelog finalization and
the version snapshot pushed earlier in the run. This repo wasn't visibly
affected — the `Update docs theme` step's `git pull --rebase` already
moves the build to the latest master — but that reliance was implicit.
The build now checks out the up-to-date branch tip (`github.ref_name`)
explicitly, via a new optional `ref` input on
`manual_release_docs.yaml`. Other entry points (on-master docs deploys,
manual dispatches) are unaffected because `ref` defaults to the
triggering ref.

Unlike here, crawlee-python lacks the theme-update step, so there the
stale ref actually broke the deployed changelog (stuck at "not yet
released") — see apify/crawlee-python#1946 for the full diagnosis.