docs: fix Sphinx build errors in maintainers and platform docs

Define substitutions for |M|, |G| and |F| in maintainers.rst
and add missing GitHub link targets to resolve Sphinx “undefined
substitution” and “unknown target name” errors.

Also fixed indentation , code-block directive and
the underline length errors.

Change-Id: I8a25a2c9a063ce04ef8e538e82d96c26ae82361c
Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
5 files changed