Documentation: filesystems: update filename extensions

Documentation: filesystems: update filename extensions

Update references to most txt files to rst files.
Update one reference to an md file to a rst file.
Update one file path to its current location.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org
Cc: Ian Kent <raven@themaw.net>
Cc: autofs@vger.kernel.org
Cc: Alexander Aring <aahringo@redhat.com>
Cc: David Teigland <teigland@redhat.com>
Cc: gfs2@lists.linux.dev
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Theodore Y. Ts'o <tytso@mit.edu>
Cc: fsverity@lists.linux.dev
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: ocfs2-devel@lists.linux.dev
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241120055246.158368-1-rdunlap@infradead.org