feat: add "web handler" feature; it must get the menu handler

this makes more sense than (potentially) adding a whole bunch of
methods to the app handler.

also fix entry point for app provider, per wj changes

docs/api/wuttaweb/handler.rst

@@ -0,0 +1,6 @@
+
+``wuttaweb.handler``
+====================
+
+.. automodule:: wuttaweb.handler
+   :members:

docs/api/wuttaweb/index.rst

@@ -8,6 +8,7 @@
    :maxdepth: 1
 
    app
+   handler
    helpers
    menus
    static