Merge branch 'nfc-convert-from-txt-to-rst'

Robert Schwebel says:

====================
here is v2 of the series converting the NFC documentation from txt to
rst. Thanks to Jonathan and Dave for the input.

Changes since (implicit) v1:

* replace code-block by more compact :: syntax

* really add the rst file to the index
====================

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>