feat: sample, documentation, package metadata and an extending skill

[kieronlanning]

Adds the remaining consumer-facing surface to the new repository.

• Package metadata: refined Description and PackageTags; the GitHub repo description, homepage and topics are set.
• Documentation: a rewritten root README plus a docs site — getting started, policy reference, type-level rules, extending — with a MkDocs nav.
• Sample: src/samples/ArchitectureSample is a small, fully working repository (Domain/Application/Persistence/Composition) with its own policy, a type-level rule enabled, and a one-line test class. Wired into the solution and the pipeline's test discovery.
• Skills: adds purview-architests-extending alongside purview-architests; the main skill now points at it and drops the overlapping detail.

Validated: dotnet test (93 tests across the package, repository and sample), dotnet csharpier check . and the full purview-build pipeline (build, lint, tests, pack, validate) are green.