We now add the htmlMfm key when relevant, store this in the database, and we see it when fetching using e.g.
curl -L -H 'Accept: application/activity+json' "$ap_id"
The `@context` of the Activity Pub message now also contains `htmlMfm: https://w3id.org/fep/c16b#htmlMfm`.
When an incomming post has `htmlMfm: true`, we will not re-parse the content.
FEDERATION.md is adapted to show the `htmlMfm` term is used.
And while add it point to this via a top-level
FEDERATION.md document as standardised by FEP-67ff.
Also add a few missing descriptions to the config cheatsheet
and move the recently removed C2S extension into an appropiate
subsection.
