Update links to documentation

Replaces all mentions of knot-resolver.readthedocs.io with www.knot-resolver.cz/documentation/latest.

Some of the links used to point to the latest documentation, which meant the latest master commit, but the current system does not really allow us to do this, so instead we link to the latest stable (which is what www.knot-resolver.cz/documentation/latest basically is).

We also cannot reliably get the documentation for a particular version in the code, so it all just points to latest as well. This may change in the future, although I do not yet have a good approach in mind (particularly, I don't want to bother our admins with nginx configuration updates for each newly released version).