Add an optional xrootd extra and use it in CI

[lobis]

Summary

• add a dedicated xrootd optional dependency extra in pyproject.toml
• update nox, docs, and contributor install instructions to opt into that extra where needed
• switch CI/docs workflows from manual mamba install xrootd setup to pip install with the extra
• lazily import the XRootD-backed classes so import fsspec_xrootd still works without the extra and points users to the install hint when accessed

Testing

• ./.venv39/bin/python -m pytest tests/test_package.py
• python3 -m compileall src tests
• isolated python3 -m venv install of -e . without xrootd, verifying import fsspec_xrootd still works and accessing XRootDFileSystem raises the new guidance

Refs #89

[lobis]

@lgray feel free to comment on the discussion at #89