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>

Documentation/filesystems/ext3.rst

@@ -0,0 +1,14 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+Ext3 Filesystem
+===============
+
+Ext3 was originally released in September 1999. Written by Stephen Tweedie
+for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
+Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
+
+Ext3 is the ext2 filesystem enhanced with journalling capabilities. The
+filesystem is a subset of ext4 filesystem so use ext4 driver for accessing
+ext3 filesystems.
+