docs: add full-site version snapshots

[yiaany]

Which issue does this PR close?

• Closes Versioned documentation #17071.

Rationale for this change

The documentation published at https://datafusion.apache.org/ currently follows
main, so it can describe behavior that differs from released DataFusion
versions.

This PR introduces a complete immutable documentation snapshot for DataFusion
55.0.0, while keeping the root site on the latest main documentation. The
approach follows the model discussed in #17071: build the complete tagged site
into one version directory and use the PyData Sphinx Theme version switcher.

What changes are included in this PR?

• Adds a site-wide version picker for Development and 55.0.0.
• Builds the exact 55.0.0 tag into /versions/55.0.0/.
• Verifies that tag 55.0.0 resolves to commit
  d5552342012888b7d1a3ab88d92e3d292fc0cde0.
• Preserves tagged documentation content, including the existing
  "has not been released yet" sentence.
• Pins release links that would otherwise point to mutable main or latest
  content.
• Preserves all existing immutable version directories during normal
  documentation deployment.
• Deletes stale current-site output without deleting archived releases.
• Uses serialized, non-force deployment pushes.
• Adds generated-site validation and regression tests.
• Documents manual snapshot publication and the initial bootstrap sequence.
• Excludes Sphinx build caches and environment-specific metadata from the
  published snapshot.
• Refuses to replace an existing assembly output directory.

This PR does not publish generated documentation, modify asf-site, backfill
older releases, add latest or stable aliases, or automate future release
publication.

Are these changes tested?

Yes. I ran:

• git diff --check
• ./ci/scripts/doc_prettier_check.sh
• bash ci/scripts/markdown_link_check.sh
• ./ci/scripts/typos_check.sh
• ./ci/scripts/license_header.sh
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• ./dev/rust_lint.sh
• 17 versioned-documentation unit and integration tests
• current documentation build with warnings treated as errors
• exact-tag 55.0.0 full-site snapshot build
• final deployment assembly and generated-site validation
• browser smoke tests over HTTP for version switching, missing-page fallback,
  mobile layout, and JavaScript-disabled release identification

The final snapshot contains no .doctrees, .buildinfo, temporary build paths,
or generated artifacts tracked in Git.

Are there any user-facing changes?

Yes.

The current documentation remains at the site root. Users can switch between:

• Development: https://datafusion.apache.org/
• 55.0.0: https://datafusion.apache.org/versions/55.0.0/

The initial 55.0.0 snapshot must be published manually.

Bootstrap coordination

This PR intentionally remains a draft until maintainers choose the initial
publication sequence.

The current asf-site branch does not yet contain versions/55.0.0/, and
deployment validation intentionally fails closed until the snapshot exists.

Could a maintainer confirm which bootstrap procedure should be used?

1. Merge a preliminary retention-only workflow change, then publish the
   snapshot manually.
2. Use a coordinated window: verify that no old documentation deployment is
   running or queued, publish the snapshot, merge this PR, and verify that the
   new deployment retains it.

I have not modified or published anything to asf-site.

I reviewed the implementation end-to-end and can explain the exact-tag build,
publication boundary, archive retention, validator, and deployment sequence
during review.