Adds the /msisdn' 3pid type and generally fleshes out what a 3pid
is and how they work.
This merges most of the work from Max Dor in https://github.com/matrix-org/matrix-doc/pull/1039
with some tweaks and additions.
According the the openapi spec, examples for responses and schemas should be
raw objects rather than being json strings. (It's unclear what non-json
examples should look like...).
The swagger UI used to support json strings, but no longer does. In short,
let's turn the json strings into their raw formats.
We're licensing hte spec under ASLv2. Add the LICENSE file, and add the
short-form to as much of the source as is practical right now (adding it to
json source is a massive pita).
