Update links to documentation

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).