[Hafnium] Hafnium documentation moved to readthedocs

Hi,

This is to inform that the Hafnium documentation is being migrated to its own ReadTheDocs project (from TF-A RTD project): https://hafnium.readthedocs.io

This primarily concerns the SPM design doc and threat model.

Hafnium documentation changes should target hafnium tree moving forward: https://git.trustedfirmware.org/hafnium/hafnium.git/tree/docs

The remaining work over the coming days is: -creating a docs watcher automatically triggering RTD builds. -polishing few leftovers originating from TF-A in migrated documents. -cleanly removing the now duplicated docs from TF-A tree.

If any question feel free to reach to the team.

Regards, Olivier.