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

Documentation/filesystems/index.rst

@@ -83,6 +83,7 @@ Documentation for filesystem implementations.
    erofs
    ext2
    ext3
+   ext4/index
    f2fs
    gfs2
    gfs2-uevents

Documentation/index.rst

@@ -171,17 +171,6 @@ implementation.
    x86/index
    xtensa/index
 
-Filesystem Documentation
-------------------------
-
-The documentation in this section are provided by specific filesystem
-subprojects.
-
-.. toctree::
-   :maxdepth: 2
-
-   filesystems/ext4/index
-
 Other documentation
 -------------------