docs: filesystems: convert ext3.txt to ReST

Nothing really required here. Just renaming would be enough.

Yet, while here, lets add a SPDX header and adjust document title
to met the same standard we're using on most docs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/26960235e3e7c972bd543f5dd59f1ef4f3a877c6.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>