build: upload html docs as part of release process

Use a URL https://notmuchmail.org/doc/latest to leave room for a
future more ambitious scheme deploying multiple versions.

This also forces the html docs to built as part of the release
process. In the future this should be updated to tolerate generating a
release without sphinx installed. This needs a new target analogous to
build-info and build-man that does nothing if sphinx is not installed.