Add references to backend_{gtk3,gtk4,wx} in docs.

These modules are not actual backends but rather serve as bases for the
corresponding agg/cairo backends, but adding them to the docs allows
referring to them from elsewhere.

Note that backend_qt is already listed; _backend_tk is currently private
but we could consider making it (or a subset of it) public like the
others (I already rely on it in mplcairo, for example).

Author: anntzer

doc/api/backend_gtk3_api.rst

@@ -8,5 +8,6 @@ dependency to building the docs.
 .. redirect-from:: /api/backend_gtk3agg_api
 .. redirect-from:: /api/backend_gtk3cairo_api
 
+.. module:: matplotlib.backends.backend_gtk3
 .. module:: matplotlib.backends.backend_gtk3agg
 .. module:: matplotlib.backends.backend_gtk3cairo

doc/api/backend_gtk4_api.rst

@@ -8,5 +8,6 @@ dependency to building the docs.
 .. redirect-from:: /api/backend_gtk4agg_api
 .. redirect-from:: /api/backend_gtk4cairo_api
 
+.. module:: matplotlib.backends.backend_gtk4
 .. module:: matplotlib.backends.backend_gtk4agg
 .. module:: matplotlib.backends.backend_gtk4cairo

doc/api/backend_wx_api.rst

@@ -7,5 +7,6 @@ dependency to building the docs.
 
 .. redirect-from:: /api/backend_wxagg_api
 
+.. module:: matplotlib.backends.backend_wx
 .. module:: matplotlib.backends.backend_wxagg
 .. module:: matplotlib.backends.backend_wxcairo