doc: move README.menu to HTML documentation

Convert README.menu to reStructured text and add it to the HTML
documentation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 83eb130..beaa64d 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -12,6 +12,7 @@
    commands
    global_data
    logging
+   menus
 
 Debugging
 ---------