Update MkDocs version to 1.6

[fyliu]

Overview

PeopleDepot would like to have this Docker image updated to MkDocs 1.6.
1.6 adds more validation functionality that's useful for CI.

Action Items

• update mkdocs version to 1.6
• possibly add an example of using the new validation (ask @fyliu for the branch it exists in)

Resources/Instructions

• See the Maintainer section in the documentation site for how to do common tasks.
• Roughly
  • go into the poetry shell
  • run poetry update
  • run the export command to write the requirements.txt file
  • rebuild the image and test
  • commit changes
• It might be a good idea to unpin the versions so we get the latest. I don't know that we need to stay with a certain version of anything.