Complete spec for notifications

[glpatcern]

So far the spec is incomplete with respect to the /notifications endpoint.

We should do the job by reverse engineering the Nextcloud and oCIS implementations, which claim to be compliant, and of course by adding cross implementation tests.

[MahdiBaghbani]

I already did a lot of this work in my STA M5 attempt (although didn't really use it that much), but it's still pretty scattered and rough
I'm happy to clean it up, turn it into a proper write up, and open a PR here
If it should go somewhere else, just let me know

[glpatcern]

A write up in this issue and/or a PR here are welcome!