Improve readability

In order to improve readability, we change the following things:

• Remove custom stylesheet
• Change ordering of tools in overview table to reflect UI (and TOC)
• Update link to repo

The custom stylesheet makes the docs use the openDesk purple color in different places, e.g., in headlines and tables. This color is so strong though, that it should only be used at very few occassions, e.g., the "openDesk" trademark. With removing the custom stylesheet, the docs fall back to the default stylesheet from the Mkdocs theme we use, which IMHO is just fine to use.