docs: add instructions for building the docs

Although building these docs is mostly done through the standard
sphinx-build command, some additional dependencies and environment
variables are required for PlantUML and Graphviz generation to work
properly.

The first section is adapted from the main TF-M documentation.

Change-Id: I670b6361e9904166a3babccbabeac655f247699a
Signed-off-by: Nik Dewally <Nik.Dewally@arm.com>
3 files changed