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 <[e-mail]>
Link: https://lore.kernel.org/r/269602[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/ext3.txt Documentation/filesystems/ext3.rst

···
+.. SPDX-License-Identifier: GPL-2.0
 
+===============
 Ext3 Filesystem
 ===============
 

Documentation/filesystems/index.rst

···
    efivarfs
    erofs
    ext2
+   ext3
    fuse
    overlayfs
    virtiofs