docs: generate the list of options

To generate the list of options, we need to generate and commit a rst
file to make all files available for ReadTheDoc.

An Hydra test ensures this generated file is up-to-date. If it is not
up-to-date, the error message explains the user how to generate it:
the user just needs to run `nix-shell --run generate-rst-options`.

docs/howto-develop.rst

@@ -46,6 +46,13 @@ documentation:
    $ make html
    $ firefox ./_build/html/index.html
 
+Note if you modify some NixOS mailserver options, you would also need
+to regenerate the ``options.rst`` file:
+
+::
+
+   $ nix-shell --run generate-rst-options
+
 Nixops
 ------