docs: networking: nfc: change to rst format

Now that the sphinx syntax has been fixed, change the document from txt
to rst and add it to the index.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

Documentation/networking/index.rst

@@ -33,6 +33,7 @@ Contents:
    scaling
    tls
    tls-offload
+   nfc
 
 .. only::  subproject and html