Provide real documentation

Instead of a single bloated readme, we should provide a real documentation as this project grows.

We should especially add information about how to use this project:

• What can be configured
• How can something be configured
• How to configure the project to be adjusted to your needs, with examples to understand the possibilities.

Of course this involves the following steps:

• Migrate readme to documentation
• Render documentation (depending on hosting / integration, see below)
• Publish documentation (depending on hosting / integration, see below)

Therefore:

• Where should we host the documentation? I prefer read the docs as they also do the rendering and stuff. There is already a webhook working out of the box: https://read-the-docs.readthedocs.io/en/latest/webhooks.html#gitlab for gitlab. Otherwise we just need a further job in our pipeline triggering the webhook: https://read-the-docs.readthedocs.io/en/latest/webhooks.html#others
• How should we write the docs? I would prefer sphinx: http://www.sphinx-doc.org/en/stable/