docs: migrate to GitHub Pages (httpware.modern-python.org)

[lesnik512]

Summary

• Replaces ReadTheDocs with GitHub Pages as the authoritative docs host, modeled on the sibling modern-di setup.
• New URL: https://httpware.modern-python.org.
• Adds .github/workflows/docs.yml, docs/CNAME, and a docs-deploy recipe in the Justfile.
• Deletes .readthedocs.yaml (clean cutover).
• Repoints all references: mkdocs.yml, pyproject.toml, CONTRIBUTING.md, planning/engineering.md. Also fixes already-dead modern-di.readthedocs.io URLs in docs/recipes/modern-di.md and two planning artifacts.

Spec: planning/specs/2026-06-08-mkdocs-gh-pages-migration-design.md

Operational prerequisites (NOT in this diff)

These must be done by a repo admin around the merge — flagging here so they don't get forgotten:

1. DNS: add a CNAME record httpware.modern-python.org → modern-python.github.io on the modern-python.org zone.
2. First-deploy bootstrap: after the first workflow run, go to Settings → Pages and set Source = Deploy from a branch, Branch = `gh-pages` / (root).
3. HTTPS: once Pages serves the custom domain, tick Enforce HTTPS in Settings → Pages.

Follow-up (separate work, not blocking merge)

• Archive the ReadTheDocs project at `readthedocs.org/projects/httpware/` so the old URL stops serving stale content.

Test plan

• Mkdocs `--strict` build succeeds locally.
• `CNAME` lands in the built site at root.
• `httpware.modern-python.org` baked into `index.html`.
• After merge: `Deploy Docs` workflow run on `main` succeeds.
• After admin completes prereqs above: `https://httpware.modern-python.org/\` loads and matches current RTD content.

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