docs: clarify instructions for running RME-enabled fw

This patch adds some documentation improvements in response to out-of-box
usability testing:

* Provide targetted links to other parts of the documentation.
* Clarify which toolchain is required.
* Clarify where the FVP binary and the bl1.bin and fip.bin files can be found
* Remove comment that cache state modelling is disabled in the known
  limitations, since it is now supported and enabled.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I0b1d889c17edb5a40c4ed81c2a58c593258c3139