Minor CMake and ReadMe edits.

Change-Id: Ib5b3fb904a0b034af57a3d4065a6183e29499f0f

 * fixing simple_platform and native build commands in the docs.
* Added link to a public forum
* table of contents fixed
* Minor doc pages refactoring related to TM
* updated API img

Change-Id: Ibf703d9e51084daeab1c3263f996008523250236

Alexander Efremov authored Mar 26, 2021 and Kshitij Sisodia committed Mar 29, 2021

Change-Id: I12e807f19f5cacad7cef82572b6dd48252fd61fd