CI: Build DocC catalog on every PR

[itsniper]

Goal

Wire swift package generate-documentation into the CI pipeline so that public API renames or broken DocC anchors fail the build, rather than drifting silently.

Context

See the investigation report at docs/investigations/swift6-concurrency-audit-2026-05-13.md, Preventive Measures. Quoting:

DocC build in CI (swift package generate-documentation) so public API renames break the docs build, not silent drift.

The library has a DocC catalog at Sources/ReliaBLE/Documentation.docc/. The swift-docc-plugin is already a package dependency. This issue is the CI-side hookup.

Deliverable

1. Add a job/step in .github/workflows/ci.yml (or a separate workflow) that runs:

   swift package generate-documentation --target ReliaBLE

2. Configure the build to fail on DocC warnings (broken links, missing anchors). The plugin supports --warnings-as-errors (or equivalent) — use it.
3. Optionally: publish the rendered DocC archive as a build artifact for download from the PR check, so reviewers can preview docs changes without checking out the branch.
4. (Future, not required for this issue) Publish to GitHub Pages on main merges.

Acceptance criteria

• A new CI step / workflow runs DocC generation on every PR.
• A PR that breaks a DocC anchor or removes a documented symbol fails this check.
• The check shows up in the PR's status checks UI.

References

• Investigation report: docs/investigations/swift6-concurrency-audit-2026-05-13.md
• Parent: NFR 11: Continuous Integration (GitHub Actions) #21 (NFR 11 — CI)