This patch introduces a set of minor bug-fixes,
aimed at increasing the quality of the rendered
documentation

It also adds a new feature of exposing cmake
variables to the documentation.

* Exposed @SPHINXCFG_TFM_VERSION@ to .rst documents
  as |TFM_VERSION|
* Upgraded tfm_its_512_flash document to comply with
  the documentation guidelines
* Fixed a typo in tfm_build_instruction document
* Reworded the cryptocell-312-runtime readme file with
  valid links to the user-manual
* Added psoc64/security/keys/readme.rst to excluded
  document-list
* Fixed an external link referencing on readme.rst

Change-Id: Ic6406fb9b59ff25f964506675a24334beeaaa738
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>