docs: convert docs to ReST and rename to *.rst

The conversion is actually:
  - add blank lines and indentation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - adjust title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/ubifs-authentication.md

@@ -417,9 +417,9 @@ will then have to be provided beforehand in the normal way.
 
 [DMC-CBC-ATTACK]     http://www.jakoblell.com/blog/2013/12/22/practical-malleability-attack-against-cbc-encrypted-luks-partitions/
 
-[DM-INTEGRITY]       https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.txt
+[DM-INTEGRITY]       https://www.kernel.org/doc/Documentation/device-mapper/dm-integrity.rst
 
-[DM-VERITY]          https://www.kernel.org/doc/Documentation/device-mapper/verity.txt
+[DM-VERITY]          https://www.kernel.org/doc/Documentation/device-mapper/verity.rst
 
 [FSCRYPT-POLICY2]    https://www.spinics.net/lists/linux-ext4/msg58710.html