Lots of documentation changes

* added a YouCanHelp directive to replace FIXMEs and encourage contributors
  to help out
* moved some bits around between the hacking howto and the codebase documents
* expanded on the stub nature of the theming howto
* tweaked some other text

docs/mgext/youcanhelp.py

@@ -0,0 +1,44 @@
+from docutils import nodes
+
+from sphinx.util.compat import Directive, make_admonition
+
+class youcanhelp_node(nodes.Admonition, nodes.Element):
+    pass
+
+class YouCanHelp(Directive):
+    has_content = True
+    required_arguments = 0
+    optional_arguments = 0
+    final_argument_whitespace = False
+    option_spec = {}
+
+    def run(self):
+        ad = make_admonition(
+            youcanhelp_node,
+            self.name,
+            ["You Can Help!"],
+            self.options,
+            self.content,
+            self.lineno,
+            self.content_offset,
+            self.block_text,
+            self.state,
+            self.state_machine)
+        ad[0].line = self.lineno
+        return ad
+
+def visit_youcanhelp_node(self, node):
+    self.visit_admonition(node)
+
+def depart_youcanhelp_node(self, node):
+    self.depart_admonition(node)
+
+def setup(app):
+    app.add_node(
+        youcanhelp_node,
+        html=(visit_youcanhelp_node, depart_youcanhelp_node),
+        latex=(visit_youcanhelp_node, depart_youcanhelp_node),
+        text=(visit_youcanhelp_node, depart_youcanhelp_node)
+        )
+    
+    app.add_directive('youcanhelp', YouCanHelp)