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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>
2 files changed