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/fymw02lywin6zmf8i4ami2lm47vwj0wk-mkdocs-0.17.5/bin/.mkdocs-wrapped passed the binary check.
- /nix/store/fymw02lywin6zmf8i4ami2lm47vwj0wk-mkdocs-0.17.5/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 0.17.5 with grep in /nix/store/fymw02lywin6zmf8i4ami2lm47vwj0wk-mkdocs-0.17.5
- directory tree listing: https://gist.github.com/270332eda9d075a9b90d1867c2326b6c
- du listing: https://gist.github.com/3e1c186f1be8c8e099b160f6d254b4e6

mkdocs: 0.17.4 -> 0.17.5