docs: Include ext4 documentation via filesystems/

The documentation for other filesystems is already included via
filesystems/index.rst. Include ext4 in the same way and remove it
from the top-level table of contents.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20210101215215.1047826-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>