doc: nav for supported boards simplified

Supported platforms section (platforms.rst) renamed to "boards.rst"
and broke into two tables instead of one (x86 arch and ARM arch).
Developer feedback was that navigating for board config options
by board architectures should be made easier.

Also updated index w/TOC to make supported board details
easier to find.

Change-Id: Ic014a720fd33ee685bde5809dc5b9e0622f5e989
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
2 files changed