Ref guide: upgrade Antora to 3.2.1

[dsmiley]

Bumps antora from 3.1.12 to 3.2.1 in gradle/libs.versions.toml.

Motivation: Antora 3.1.x can't use a git worktree as a local content source (it requires .git to be a directory), so buildLocalAntoraSite fails with "Local content source must be a git repository". Antora 3.2 supports linked worktrees as content sources.

Validated

• buildLocalAntoraSite and checkSiteLinks pass from a git worktree, with the search index and search UI present. (The Lunr extension stays at 1.0.0-alpha.10.)
• Node 22 already satisfies Antora 3.2's minimum of Node 20.
• buildOfficialAntoraPlaybookYaml generates the same playbook. It is a template copy and doesn't exercise Antora.

Needs validation

• buildOfficialSite. It clones apache/solr (all branch_* except branch_*x) and apache/solr-operator (release-*), so it builds older branches' content with the new Antora. I could not run it locally because of corporate TLS interception, so please run it.
• Playbook and attribute parsing. Antora 3.2.0-alpha.9 changed YAML parsing of attribute and site key values (quotes are no longer stripped). The local build was fine, but the official playbook and the Operator component are untested.
• Release flow (releaseWizard, setOfficialAntoraYaml) is untested.
• A build from a normal (non-worktree) clone.

Concerns

• Antora 3.2.0 was released 2026-08-29 and 3.2.1 on 2026-10-01, so it is recent.
• The build logs a new-looking error: rule-based-authorization-plugin.adoc:427: dropping cells from incomplete row detected end of table. It does not fail the build. It looks like a pre-existing content issue, since SOLR-18464 changed that table to cols="1,1,1,1". I didn't confirm that the upgrade is what surfaces it.
• The default UI bundle and Lunr extension versions are unchanged. A newer Lunr extension exists (1.0.0-alpha.13).

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