Use ExDoc

[xxdavid]

The documentation of brod is much nicer now when you use ExDoc (I love it!). I think it would be great if kafka_protocol used it as well.

Aside from UI/UX, it would make links (particularly for types) from brod's documentation to kafka_protocol's one to work better. ExDoc uses a different format of anchors than EDoc does, so for example the link https://hexdocs.pm/kafka_protocol/kpro.html#t:connection/0 opens the page at the top and does not jump to directly to the connection/0 type so one has to find it manually.