I replaced every occurrence of codimd with hedgedoc. There were still
some stray hackmd references, so I replaced them as well. This includes
using hedgedoc/hedgedoc instead of hedgedoc/server.

I removed the kubernetes instructions, since they were referring to
a helm chart that does not exist anymore.

There are two references to hackmd left. The first one is in the
migration guide from hackmd-docker to hedgedoc-container. The other one
is in a migration guide that points to
https://github.com/hackmdio/migration-to-0.5.0. Since the 0.5.0 version
is quite old, these instructions were removed.

Deprecated CODIMD_REPOSITORY in favor of HEDGEDOC_REPOSITORY. People
using CODIMD_REPOSITORY will now see an error in their docker build
prompting them to change to HEDGEDOC_REPOSITORY.

We are now using chat.hedgedoc.org as the matrix link.
Signed-off-by: Hugo Peixoto <hugo.peixoto@gmail.com>