Documentation: filesystems: convert vfat.txt to RST

Converts vfat.txt to the reStructuredText format, improving presentation
without changing the underlying content.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
-----------------------------------------------------------
Changes in v3:
Removed unnecessary markup.
Removed section "BUG REPORTS" as recommended by the maintainer.

Changes in v2:
Refactored long lines as pointed out by Jonathan
Copied the maintainer
Updated the reference in the MAINTAINERS file for vfat

I did not move this into admin-guide, waiting on what the
maintainer has to say about this and also about old sections
in the text, if any.

Link: https://lore.kernel.org/r/20191223010030.434902-1-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -48,3 +48,4 @@ Documentation for filesystem implementations.
 
    autofs
    virtiofs
+   vfat

MAINTAINERS

@@ -17356,7 +17356,7 @@ F:	drivers/mtd/nand/raw/vf610_nfc.c
 VFAT/FAT/MSDOS FILESYSTEM
 M:	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
 S:	Maintained
-F:	Documentation/filesystems/vfat.txt
+F:	Documentation/filesystems/vfat.rst
 F:	fs/fat/
 
 VFIO DRIVER