docs: reorganize the navigation

[jmineau]

The top bar becomes Getting Started, Tutorials, User Guide, API Reference, and Development. It was eight entries, two of them under a "More" menu.

• Coming From Another STILT moves under the User Guide (guides/migration/). The landing page tile now says "Coming from another STILT?" and links to that index.
• Development holds three pages: Architecture, STILT-R Parity (split out of the old Development page), and Roadmap.
• How PYSTILT tracks finished work is merged into Architecture. The two pages said much of the same thing, so the repeated paragraphs are gone and the facts only the old page had (workdir names, killed runs, atomic writes, config-order variants) are in Architecture. The Internals section is removed. The finished-work label keeps the cross references from the stiltctl page and the project reference.
• The guide to writing docs (voice, API pages, how examples run) moves from the Development page to CONTRIBUTING.md, in its own commit. It tells contributors how to write text for the repository, so it does not need to be on the site. Drop that commit to keep it on the site.

Old addresses change (roadmap.html, development.html#stilt-r-parity, migration/*, advanced/*). README, AGENTS.md, CONTRIBUTING.md, and the Unreleased changelog entry now name the new ones. Nothing else in the other packages links to them.

sphinx-build -W passes on both commits.

Open in this package: #229 edits docs/migration/stilt_r.rst, docs/migration/x_stilt.rst, docs/advanced/architecture.rst, and docs/roadmap.rst. Git follows the renames, so whichever merges second rebases onto the first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.