chore(site): publish via CI & remove tracked site/; implement gh-pages deployment

[ShawnMcKee]

This PR adds a GitHub Actions workflow that builds MkDocs and publishes to gh-pages and removes the checked-in site/ directory.

Key changes:

• Add .github/workflows/deploy-gh-pages.yml to build & deploy to gh-pages on pushes to master.
• Update .github/scripts/verify-site-scripts.sh to support doc-only verification when site/ is not present and verify changed scripts when relevant.
• Add .gitignore to ignore site/ and .venv
• Remove site/ directory from the repository (deleted tracked files) — the published site will now be produced by CI.

Review notes:

• CI will now build & publish the site/ directory to gh-pages for each push to master and manually via workflow dispatch.
• The verify/site parity script now runs against the freshly-built site in CI, and locally you can run: mkdocs build && ./.github/scripts/verify-site-scripts.sh to validate.
• The PR also includes persona UX improvements (banners) and a website management doc in docs/web-site-management.md.

If we decide to keep site/ tracked in the repo, we can revert removal. This PR is intended to move to CI-driven site publishing for a cleaner content workflow.