Docs: Create new area for developers.

We need some "Part" for developers. Currently, it's named
"Part 4: Developer's Zone".  But we should come up with a
better name soon.

Moved the codebase docs in there for starters.

docs/source/index.rst

@@ -44,7 +44,6 @@ MediaGoblin website.  It is written for site administrators.
    siteadmin/relnotes
    siteadmin/theming
    siteadmin/plugins
-   siteadmin/codebase
 
 
 .. _core-plugin-section:
@@ -74,6 +73,17 @@ This guide covers writing new GNU MediaGoblin plugins.
    pluginwriter/api
 
 
+Part 4: Developer's Zone
+========================
+
+This chapter contains various information for developers.
+
+.. toctree::
+   :maxdepth: 1
+
+   devel/codebase
+
+
 Indices and tables
 ==================