[Website] Publish versioned Paimon C++ documentation - #38

The website currently publishes only main-branch C++ documentation at docs/cpp/. Publish the stable release at that URL, development documentation at docs/cpp/main/, and archived series at docs/cpp/X.Y/, using the version list maintained on paimon-cpp main.

Full labels such as 0.4.0 (stable) select release-0.4; archived 0.3.0 selects release-0.3. Promoting the upstream version list for 0.4 requires no website version configuration changes. Each build sets PAIMON_DOCS_VERSION for the switcher and warning banner, installs the branch's requirements, and receives the shared version list from main.

Stage all versions before replacing the published tree, preserving Sphinx assets and nested version directories. Missing branches, invalid metadata, or failed builds stop publication. README is unchanged.

Related: apache/paimon-cpp@2c6f749

Dependency: apache/paimon-cpp#496 provides version navigation on release-0.3 and must land before publishing that branch with navigation.

Validation:

• bash -n .github/workflows/cpp-docs.sh and git diff --check passed, including after rebasing onto current master.
• Temporary Git repositories with simulated Doxygen/Sphinx builds covered 0.3 stable plus main, 0.4 promotion with 0.3 archival, patch releases, asset preservation, obsolete output removal, build failures, missing branches, and invalid metadata.
• Full upstream documentation builds were not run.