Add CI check for mkdocs build

[oskooi]

Added .github/workflows/docs.yml:

• Triggers only on changes to doc/**, mkdocs.yml, or the workflow itself (keeps it off unrelated pushes).
• Checks out, sets up Python 3.11, installs doc/requirements.txt, and runs mkdocs build
• This catches broken doc builds on every PR without having to compile Meep (it builds from the committed Markdown tree, which already includes the generated Python_User_Interface.md).
• A comment documents the one thing it can't do — regenerate the API doc from docstrings (that needs a Meep build; make docs locally covers it).

Not implemented (possible feature to add separately)
Catching a stale committed Python_User_Interface.md (docstrings changed but the generated file wasn't regenerated). This would require running make docs inside build-ci.yml (where Meep is already compiled) and failing on a non-empty git diff.