Which issue does this PR close?

Rationale for this change

The User Guide has no rendered API reference today. Readers can browse the
narrative pages but have to drop into source code to look up class and method
signatures.

What changes are included in this PR?

Generates Javadoc at docs build time and embeds it under /api/ in the
Sphinx output. A new "API Reference" page in the User Guide links into it.

• pom.xml — adds maven-javadoc-plugin 3.11.2, configured to exclude the
  generated protobuf packages (datafusion_common.*,
  org.apache.datafusion.protobuf.*) so the published reference only covers
  hand-written API.
• docs/build.sh — invokes ./mvnw javadoc:javadoc from the repo root and
  stages target/reports/apidocs/ into docs/source/_extra/api/ before
  sphinx-build runs.
• docs/source/conf.py — adds html_extra_path = ["_extra"] so Sphinx
  copies the staged tree verbatim into the output, plus
  exclude_patterns = ["_extra"] so the Javadoc-emitted .md files
  (legal/jquery.md, etc.) aren't scanned as Sphinx sources under -W.
• docs/source/user-guide/api-reference.md — new page in the User Guide
  toctree linking to api/index.html. The link is a raw HTML anchor with a
  comment explaining why (MyST treats relative .html Markdown links as
  cross-references and fails the strict build).
• .gitignore — ignores the new staging directory.
• docs/README.md — documents the new JDK + first-run network requirement
  for the docs build.

Are these changes tested?

The docs build itself is the verification. ./docs/build.sh now produces a
single tree at docs/build/html/ containing the existing Sphinx pages plus
/api/index.html and the surrounding Javadoc, and exits 0 under
sphinx-build -W with no warnings. The exclusion of generated protobuf
packages is verified by directory absence checks in the build output.

Are there any user-facing changes?

Yes — the published User Guide gains an "API Reference" page and a
browsable Javadoc tree at api/.