Define the supported rcParams as code

Until now, the parameter definition was dispersed:
valid names and defaults are loaded from the
canonical `matplotlibrc` data file. Docs are only
available as comments in there. Validators are
attached ad-hoc in `rcsetup.py`.

This makes for a more formal definition of parameters,
including meta-information, like validators, docs in
a single place. It will simplify the rcParams related
code in `matplotlib.__init__.py` and `matplotlib.rcsetup.py`.
It also enables us to generate sphinx documentation
on the parameters.

Author: timhoffm

galleries/users_explain/customizing.py

@@ -202,6 +202,13 @@ def plotting_function():
 plt.show()
 
 # %%
+#
+# Description of rcParams
+# =======================
+#
+# .. include:: /users/_rcparams_generated.rst
+#
+#
 # .. _customizing-with-matplotlibrc-files:
 #
 # The :file:`matplotlibrc` file