doc: move README.fdt-overlays to HTML documentation

Convert doc/README.fdt-overlays to doc/usage/fdt_overlays.rst.

Edit the text for readability.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 57bf4fe..ada0df0 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -2,7 +2,9 @@
 ==========
 
 .. toctree::
+   :maxdepth: 1
 
+   fdt_overlays
    netconsole
 
 Shell commands