Spec AS registration format

Currently the spec leaves the format of AS registration files completely
undefined. In fact, the current verbiage doesn't even make it completely clear
that the registration is an actual on-disk file. In practice, of course, synapse
requires a yaml registration file. The spec needs to clarify that either:

1. Registration is an entirely implementation-specific thing where
implementations should expect the following set of fields.
2. Registration is a physical file with a specific format (yaml, JSON, toml, etc.).

If we go for option 2, I would *strongly* push that we not enshrine any text
format other than JSON in the spec and that yaml not be an acceptable format for
this.

This issue would also be resolved by matrix-org/matrix-spec-proposals#3807.

Read the current registration wording in the Matrix specification and matrix-org/matrix-spec-proposals#3807 first. Resolve whether registration is implementation-specific or a physical file format, then update the specification so the chosen behavior and required fields are explicit.

Written by the indexing model from the issue text.