Update mkdocs & plugins and add auto-updateing with dependabot

[J0B10]

I saw that there was a security vulnerability in mkdocs and therefore updated all dependencies.

CVE-2021-40978 doesn't expose a risk to this repo as it only occurs when using mkdocs serve / mike serve, but for users that use it locally for editing & debugging the wiki it could be a huge threat especially if they are in a public network (eg. universitys, scools) as it allows access to their local files.

Therefore it should be fixed ASAP.

To faster resolve such issues in the future I added auto-updates with dependabot.
It will open a Pull-Request if any of the used pip modules has an update available.
This should also help keeping track of new updates as it's easy to forget about them, especially for the documentation.