Default docs to dev version; drop stable from version selector

[knutambot]

What

• docs/make.jl: pass versions=["dev" => "dev", "v#.#"] to deploydocs so the gh-pages root redirects to ./dev/ and the outdated-version banner (DOCUMENTER_STABLE) points at dev instead of stable.
• Released versions remain reachable via v#.# symlinks (e.g. v0.1, v0.2).
• canonical is left unchanged (bare root URL). Documenter's canonical_url builds each page's canonical link as canonical + page path, with no version segment at all — it's already version-agnostic regardless of which version the root redirects to, since the same make.jl runs for every version build (dev and tagged releases alike). Pointing it at /dev would have made every released version's pages claim .../dev/... as their canonical URL too, which is wrong.

Backwards compatibility for /stable/

A companion one-time fix on the gh-pages branch (#24) points the existing stable symlink at dev instead of a version directory — the same mechanism Documenter already uses for version aliases. Old /stable/<path> links, including deep links and assets, keep working transparently. Future deploydocs runs (with the versions setting in this PR) regenerate versions.js/index.html but do not touch the stable symlink, so that fix persists going forward.

Testing

• Pkg.test(): all suites pass (350+ assertions).
• docs/make.jl built successfully locally (deployment step auto-skipped outside CI, as expected); verified the built canonical link stays version-agnostic (.../MaterialModelsBase.jl/<page>, no /dev/ segment).
• Reviewed with an independent Codex read-only review (plan and final diff); no unresolved findings.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (2defce2) to head (5dbf8e4).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   95.09%   92.28%   -2.82%     
==========================================
  Files           5        6       +1     
  Lines         306      350      +44     
==========================================
+ Hits          291      323      +32     
- Misses         15       27      +12     

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