Documentation: filesystems: convert vfat.txt to RST

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>
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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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>