Replace the current stack of hugo templates with a towncrier invocation. The main advantage of this is that it means that the "Changes since last release" section is consistent with the changelogs for the actual releases.
This also changes the release process so that the changelog is generated before tagging, which means that the thing tagged v1.5 is actually the v1.5 spec.
Fixes#908.
Stick a `definition-` on the front of the autogenerated anchors for definition
blocks.
This solves a problem where, for example,
https://spec.matrix.org/unstable/application-service-api/#registration could
refer to either the "Registration" section or the `Registration` definition
therein.
(These anchors are relatively recent: they were added in #1191.
