Docs: Refresh the Crypto service integration guide

The TF-M Crypto service integration guide needs to be
refreshed to reflect newer changes that have been
merged over time. Also, introduce some useful references,
cross-referencing when needed, plus add support in conf.py
for newer versions of Sphinx.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I2bcbc38e5940f042b516ab44355a7ad2354dac5f
9 files changed