doc: Refactor headings to follow the official Sphinx style guide

For headings, follow the Sphinx style guide defined at:
https://documentation-style-guide-sphinx.readthedocs.io/en/latest/style-guide.html#headings



Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: If63dda1b587fcd6a402b32288680596b1d900a8a