Doc: convert markdown files to rst

The official documentation format for TF-M is becoming Restructured
Text. This change converts the existing .md files to the new format.

Due to support for documentation of external code (/lib) md support is
kept.

Change-Id: I3c1aef5d22442e0c7c362a40a8d344a266dc0940
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>