Improve documentation location/technology

[karottenreibe]

This is a summary ticket for all documentation improvements.

• I'd also prefer it, if the documentation were in the top-level README.md file of this repo as that's where users will be looking for it (plus this makes it easier for us to find it as well ;-))
• The GitHub Wiki should be removed entirely as it's outdated.
• We should use grip to render our documentation to a HTML file. Here's a sample Travis CI config that does this. The advantage is that it looks exactly like on GitHub, making it much easier to edit the documentation.

Should fix #53 .

[karottenreibe]

CC @salsolatragus

[karottenreibe]

Migrated as TS-21149