Doc: move API docs in their own chapter and add more of it.

doc/extdev/index.rst

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+.. _dev-extensions:
+
+Developing extensions for Sphinx
+================================
+
+Since many projects will need special features in their documentation, Sphinx is
+designed to be extensible on several levels.
+
+This is what you can do in an extension: First, you can add new
+:term:`builder`\s to support new output formats or actions on the parsed
+documents.  Then, it is possible to register custom reStructuredText roles and
+directives, extending the markup.  And finally, there are so-called "hook
+points" at strategic places throughout the build process, where an extension can
+register a hook and run specialized code.
+
+An extension is simply a Python module.  When an extension is loaded, Sphinx
+imports this module and executes its ``setup()`` function, which in turn
+notifies Sphinx of everything the extension offers -- see the extension tutorial
+for examples.
+
+The configuration file itself can be treated as an extension if it contains a
+``setup()`` function.  All other extensions to load must be listed in the
+:confval:`extensions` configuration value.
+
+.. toctree::
+
+   tutorial
+   appapi
+   envapi
+   builderapi
+   markupapi
+   domainapi
+   nodes