ci: add docs build check on pull requests

[zurfjereluhmie]

• Adds a docs-check job to CI that runs mkdocs build --clean --strict as a dry run on pull requests
• Uses dorny/paths-filter@v4 to only trigger when docs-related files change (docs/, mkdocs.yml, src/, .github/workflows/docs.yml)
• The ci-success gate job now includes docs-check in its dependency list with allowed-skips so it does not block PRs that don't touch documentation
• Catches syntax errors, missing references, and broken docs before they hit main

How it works

1. On every PR, the docs-check job runs and checks if any docs-related files changed
2. If yes: installs dependencies and runs mkdocs build --strict (fails on warnings)
3. If no: the job is skipped and ci-success allows it via allowed-skips