Update mkdocs base image

[one000mph]

It looks like the mkdocs image hasn't been updated for awhile. I'd like to use a newer version 3.1 to use the new toc_depth feature which will allow us to have a prettier & more customizable Table Of Contents. I can probably do the update to the docker image if ops is busy, but totally understand if this needs gravitational ops for security.

[benarent]

I'm going to look into this today.

[one000mph]

along with this it seems like code highlighting is not enabled via the codehilite extension. I imagine this is an issue that code be fixed in the Dockerfile.

https://squidfunk.github.io/mkdocs-material/extensions/codehilite/#installation

Looks like it wants pip install pygments