docs: add CI deployment examples - #4158

Summary

• add a GitHub Actions example for publishing Project Pages with mkdocs gh-deploy
• add a GitLab Pages example using mkdocs build --site-dir public
• mention installing third-party themes/plugins before the deploy/build commands

Fixes #2144.

Verification

• git diff --check
• focused snippet verifier checked the new GitHub Actions and GitLab Pages references and commands

Note: I attempted a strict docs build with .venv-codex/bin/python -m mkdocs build --strict --site-dir /private/tmp/mkdocs-osc-site, but the local venv is missing pymdownx, so MkDocs aborted during configuration loading before rendering docs.