Olm and Megolm should be specced in the actual spec

Rather than just on gitlab.matrix.org[[1]](https://gitlab.matrix.org/matrix-org/olm/-/blob/master/docs/olm.md)[[2]](https://gitlab.matrix.org/matrix-org/olm/-/blob/master/docs/megolm.md).

~Additionally, the spec [currently has a broken link](https://spec.matrix.org/v1.6/client-server-api/#mmegolmv1aes-sha2) for the Megolm spec.~ duplicate of https://github.com/matrix-org/matrix-spec/issues/1555