mkdocs: 0.17.3 -> 0.17.4 (#41988)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/mkdocs/versions.

These checks were done:

- built on NixOS
- /nix/store/jvkm61ryls5w90v32pisdy2w9hv4sjjb-mkdocs-0.17.4/bin/.mkdocs-wrapped passed the binary check.
- /nix/store/jvkm61ryls5w90v32pisdy2w9hv4sjjb-mkdocs-0.17.4/bin/mkdocs passed the binary check.
- 2 of 2 passed binary check by having a zero exit code.
- 2 of 2 passed binary check by having the new version present in output.
- found 0.17.4 with grep in /nix/store/jvkm61ryls5w90v32pisdy2w9hv4sjjb-mkdocs-0.17.4
- directory tree listing: https://gist.github.com/51f000d43eea93f02bf57debed7ac8bc
- du listing: https://gist.github.com/6f535d35f7a3c2d0e393e4e0fd0682c8