Slightly reorganize api docs.

- Move _enums to _enums_api, consistently with other modules, and
  move it to the end (together with the other private module, _api).

- Group the various GUI backends together based on the GUI toolkit.
  (It seems a bit silly to have so many pages which all refer to
  backends with no generated docs anyways, and even if we could easily
  generate docs for them on CI we would not want to document all 4
  mostly indentical copies of qt{4,5}{agg,cairo} on 4 different pages.)

  Also use `.. module::` entries to fix broken references.

Author: anntzer

doc/api/_enums.rst doc/api/_enums_api.rstdoc/api/_enums.rst renamed to doc/api/_enums_api.rst

@@ -0,0 +1,12 @@
+**NOTE** These backends are not documented here, to avoid adding a dependency
+to building the docs.
+
+:mod:`matplotlib.backends.backend_gtk3agg`
+==========================================
+
+.. module:: matplotlib.backends.backend_gtk3agg
+
+:mod:`matplotlib.backends.backend_gtk3cairo`
+============================================
+
+.. module:: matplotlib.backends.backend_gtk3cairo

doc/api/backend_gtk3_api.rst

@@ -0,0 +1,22 @@
+**NOTE** These backends are not documented here, to avoid adding a dependency
+to building the docs.
+
+:mod:`matplotlib.backends.backend_qt4agg`
+=========================================
+
+.. module:: matplotlib.backends.backend_qt4agg
+
+:mod:`matplotlib.backends.backend_qt4cairo`
+===========================================
+
+.. module:: matplotlib.backends.backend_qt4cairo
+
+:mod:`matplotlib.backends.backend_qt5agg`
+=========================================
+
+.. module:: matplotlib.backends.backend_qt5agg
+
+:mod:`matplotlib.backends.backend_qt5cairo`
+===========================================
+
+.. module:: matplotlib.backends.backend_qt5cairo

doc/api/backend_gtk3agg_api.rst

@@ -0,0 +1,16 @@
+
+:mod:`matplotlib.backends.backend_tkagg`
+========================================
+
+.. automodule:: matplotlib.backends.backend_tkagg
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
+:mod:`matplotlib.backends.backend_tkcairo`
+==========================================
+
+.. automodule:: matplotlib.backends.backend_tkcairo
+   :members:
+   :undoc-members:
+   :show-inheritance: