Feature/ documentation site with type-checked examples

[stivens]

Motivation

All of CaseComplete's documentation lived in one long README.md. This PR adds a proper site, a landing page plus docs split into pages, in the style of Chimney's docs. It's hosted on GitHub Pages at https://stivens.github.io/CaseComplete/.

The library's main selling point is a compile error, so the site is built with mdoc. Every snippet is compiled against the current library, and the error examples (mdoc:fail) show real compiler output generated at build time. Docs can no longer silently drift from the code.

That paid off straight away: the README quick start never compiled. doobie has no Put[java.time.Year], so fr"release_year = $year" didn't type-check. The examples now use Int years.

What's included

Site (website/)

• Landing page. The hero shows the real missing-field compile error. Below it:
  • a plain A => B function compared with a CaseComplete[A, B] parameter
  • the editor demo GIF
  • the features
  • install instructions
• Docs pages:
  • Getting started
  • Why not pattern matching?
  • API reference, including a section showing each compile error the library reports
  • Type aliases
  • Compatibility
• Theme: MkDocs Material with a custom light/dark palette, IBM Plex fonts, a logo and a favicon (website/docs/assets/).
• Shared fixtures: the movie example types live in website/src/main/scala/examples/movies.scala, so several pages can import them instead of repeating the setup.

Build

• project/plugins.sbt adds sbt-mdoc 2.9.2, which supports sbt 2.
• build.sbt gets a new docs project:
  • It depends on casecomplete.jvm and adds doobie for the examples.
  • It is not part of root's aggregate, so sbt test and MiMa are unchanged.
• New val latestRelease = "1.0.0". It's used both for the MiMa baseline and for @VERSION@ in the docs, so the site never advertises a version that isn't on Maven Central yet.

CI (.github/workflows/docs.yml)

• Build: runs sbt docs/mdoc and then mkdocs build --strict on PRs and pushes that touch website/, src/main/, build.sbt, project/ or the workflow itself. A broken snippet fails the PR.
• Deploy: pushes to main upload the site and deploy it to GitHub Pages.
• sbt cache: the workflow uses its own sbt cache key. With ci.yml's key, whichever workflow finished first would decide what's in the cache.

README

• Cut down to a short pitch, the GIF, install instructions and the quick start, with links to the site.
• The GIF moved from screenshots/ to website/docs/assets/.

Preview locally

sbt docs/mdoc          # or "docs/mdoc --watch"
pip install -r website/requirements.txt
mkdocs serve -f website/mkdocs.yml

After merging

• Set Settings → Pages → Source to GitHub Actions (or run gh api -X POST repos/stivens/CaseComplete/pages -f build_type=workflow). The first deploy fails until this is done.
• Set the repo's Website field to https://stivens.github.io/CaseComplete/.