Improve front-facing documentation
Improve front-facing documentation

* Cleanup and make homepage docs more concise and direct
* Highlight use of the TF Lite Delegate, guides and
  AI Tech talk

Signed-off-by: James Conroy <ce41d179dd5a8468f6c1c79fd3299ea93291d455@arm.com>
Change-Id: I7d0221bf804dd1769568beccbca93d23cdba24b6

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn