Affects the title and the table of contents. We can't realistically alter just the table of contents, but the table of contents is generated from this header.
Fixes https://github.com/matrix-org/matrix-doc/issues/1800
As per [MSC1452](https://github.com/matrix-org/matrix-doc/issues/1452)
Fixes https://github.com/matrix-org/matrix-doc/issues/1254
Although MSC1452 focuses on just the warnings part of the server notices, the base for notices has not been established in the spec. This commit adds the needed support to be able to handle notices.
No intentional divergences from the proposal are included in this changeset. There are a few additions which are used in practice although not defined in the proposal, such as who is responsible for aesthetics, sending notices, and other misc rules.
