docs: convert dax.txt to rst

Change the file extension and add the rst constructs to integrate this
doc to the documentation infrastructure and take advantage of rst
features.
Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Link: https://lore.kernel.org/r/20210531130515.10309-1-igormtorrente@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -77,6 +77,7 @@ Documentation for filesystem implementations.
    coda
    configfs
    cramfs
+   dax
    debugfs
    dlmfs
    ecryptfs