Use latest versions of mkdocs and material theme

[ThiefMaster]

While one can easily work around this by specifying a specific version in requirements.txt (e.g. indico/indico-user-docs@3c1d007), it would be nice if the Dockerfile used the latest version of the mkdocs-material image by default (either :latest or manually kept up to date so people locking the tag in their action always get a specific version)

An action setting to specify the mkdocs/material versions would be even nicer of course.