docs: extract mkdocs deps to requirements.txt and validate build on PRs

[ci7rix]

The documentation dependencies (mkdocs-material, mdx-truly-sane-lists, mike) were hardcoded in the docs.yml workflow, with nothing to install them locally. This PR moves them into docs/requirements.txt and points the workflow at it, so CI and local setups share a single source of truth.

It also adds a pull_request trigger filtered on docs/**, mkdocs.yml and the workflow, along with a mkdocs build --strict step, so a broken docs build gets caught at PR time rather than after merge. The mike deploy steps now sit behind if: github.ref == 'refs/heads/main', which keeps deployment on main while letting PRs run the build without publishing anything.

Finally, it enables content.code.copy in mkdocs.yml. The code blocks had no copy button, which made commands awkward to select by hand and prone to picking up stray characters.

[clementnuss]

thanks! that's helpful 🙃