Split versioned docs from the main site

[thatbudakguy]

Summary

Fixes #302 without splitting the docs into a separate repo/site.

PR #294 added mike for versioned docs, but since there was only one mkdocs.yml/docs_dir/nav, mike ended up versioning the entire site — blog, about page, showcase, etc. — not just the technical documentation. Every version directory on gh-pages (4.x/, 5.x/) duplicated the whole site, and because CI only auto-deployed on push to main, keeping old version branches' unversioned content in sync required manually checking out e.g. 4.x and running mike deploy by hand. This is visible in git history: showcase/index.md/community.md drifted between main and 4.x, and a blog post had to be cherry-picked onto both branches to appear on both.

This PR keeps everything in this one repository by splitting the single MkDocs config into three files and giving the two "sites" disjoint content directories and deploy targets on gh-pages:

• mkdocs-base.yml (new) — shared theme/markdown config, inherited via MkDocs's INHERIT key.
• mkdocs.yml (rewritten) — the unversioned main site (Home, About, Community, Showcase, Blog, and a new Release Calendar nav item), built from docs/. Deployed straight to the root of gh-pages.
• mkdocs-docs.yml (new) — the versioned technical documentation, built from a new top-level documentation/ folder (moved out of the old docs/docs/). Deployed by mike under a /docs/ prefix (via mike's deploy_prefix setting), so each GeoBlacklight version gets its own copy of just the docs — not the whole site.

docs/docs/releases.md ("Release Calendar", a cross-version GBL v3–v6 compatibility matrix) moved to the unversioned site as a top-level nav item, since it isn't specific to one version.

CI (.github/workflows/ci.yml) now:

• Runs mike deploy --config-file mkdocs-docs.yml on every push to main or any *.x branch, automating what was previously a manual per-branch step.
• Builds and deploys the main site to the gh-pages root only on main, via peaceiris/actions-gh-pages with keep_files: true so it never disturbs the docs/ directory mike manages (or CNAME, or the legacy 4.x//5.x/ directories from before this change, which are left as stale-but-harmless).

Not included in this PR (follow-ups)

• Backporting to origin/4.x: GitHub Actions runs the workflow file as committed on the ref being pushed, so the new "deploy on push to a version branch" trigger won't do anything for 4.x until this same restructuring (new configs + documentation/ layout) is also committed onto that branch. Planned as a small follow-up PR targeting 4.x after this merges.
• One-time mike set-default --config-file mkdocs-docs.yml --push latest: creates the /docs/ landing-page redirect. Plain mike deploy doesn't create this on its own, so it needs to be run once by hand after the first real deploy from this PR.
• Cleaning up the stale 4.x/, 5.x/, root versions.json/latest/index.html already on gh-pages from before this change — left alone; they'll go stale but won't 404.

Test plan

• mkdocs build --config-file mkdocs.yml --strict and mkdocs build --config-file mkdocs-docs.yml --strict both build cleanly.
• Confirmed the moved metadata.md's read_csv table (via table-reader) still renders after the path fix.
• Ran a real, no-push mike deploy in a scratch clone and confirmed deploy_prefix: docs produces gh-pages:/docs/5.x/, /docs/latest, /docs/versions.json while leaving the existing root versions.json/index.html/4.x//5.x/ untouched.
• After merge: watch the Action run once, then confirm https://geoblacklight.org/ and https://geoblacklight.org/docs/latest/ both resolve, and the version-switcher dropdown lists 5.x/4.x and switches correctly.
• Run the one-time mike set-default command noted above.
• Open the 4.x backport PR.

🤖 Generated with Claude Code

[karenmajewicz]

Tested this locally - I can run both the main site and the docs site separately. I don't see a link from the main site to the docs site, though. Is it supposed to show up in the nav?