Fix the moved Media Types section

A lot of this content is outdated, unnecessary, or otherwise
in need of changing.

* Be more clear about the OpenAPI vs IANA registries.
* We do not need to list out all of GitHub's custom media types,
  and it's better to link to the IANA registry for examples.
* The RFC for registering media types should be informative,
  not normative, if we even need it at all.
* We can merge the Sequential Media Types section into the section
  on Complete vs Streaming Content, and reorder those sections
  a bit to read more smoothly.
* Dump a lot of text that is now better addressed by the
  "Parsing and Serializing" section under the Schema Object.