mkdocs serve starts with error page

[schlomo]

When I run make serve to browse the documentation the "start page" is actually an error page:

The reason is probably the lack of an index.md or maybe README.md in the docs subdirectory.

According to mkdocs/mkdocs#321 there is no simple way to avoid duplicating the readme info (otherwise this would have been a PR).

Goal: Meaningful infos on the "start page" of the docs

I'd also suggest to mention "proudly sponsored by Bonial" or something like that

[martinohmann]

mkdocs.yml is meant for configuring documentation generation in our internal backstage instance. We can add an index.md at one point which should get rid of the 404, but I don't see this as high priority at the moment.