btrfs-progs: docs: group developer documentation

Add new directory and move ther all exisisting documentation meant for
developers.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/index.rst

@@ -53,6 +53,16 @@ Welcome to BTRFS documentation!
    Source-repositories
    Contributors
 
+.. toctree::
+   :maxdepth: 1
+   :caption: Developer documentation
+
+   dev/Experimental
+   dev/dev-btrees
+   dev/dev-btrfs-design
+   dev/dev-send-stream
+   btrfs-ioctl
+
 .. toctree::
    :maxdepth: 1
    :caption: TODO
@@ -60,9 +70,4 @@ Welcome to BTRFS documentation!
    Quick-start
    Interoperability
    trouble-index
-   Experimental
-   btrfs-ioctl
    DocConventions
-   dev-send-stream
-   dev-btrees
-   dev-btrfs-design