docs(spm): refresh FF-A SPM design doc

- Move manifest binding doc as a dedicated SPM doc section.
- Highlight introduction of an EL3 FF-A SPM solution.
- Refresh TF-A build options.
- Refresh PE MMU configuration section.
- Add arch extensions for security hardening section.
- Minor corrections, typos fixes and rephrasing.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I2db06c140ef5871a812ce00a4398c663d5433bb4