setup.py, CONTRIBUTING.rst: Recommend using tox to generate man pages

Simplify our docs, by requesting users to run `tox -e man` to update man
pages, instead of manually installing `click-man` and running the
command manually.

CONTRIBUTING.rst

@@ -1259,14 +1259,9 @@ into the ``setup.py``, as such, whenever the frontend command line
 interface changes, the static man pages should be regenerated and
 committed with that.
 
-To do this, first ensure you have ``click_man`` installed, possibly
-with::
+To do this, run the following from the the toplevel directory of BuildStream::
 
-  pip3 install --user click_man
-
-Then, in the toplevel directory of buildstream, run the following::
-
-  python3 setup.py --command-packages=click_man.commands man_pages
+  tox -e man
 
 And commit the result, ensuring that you have added anything in
 the ``man/`` subdirectory, which will be automatically included

setup.py

@@ -100,10 +100,9 @@ def check_for_bwrap():
 # Man pages are automatically generated however it was too difficult
 # to integrate with setuptools as a step of the build (FIXME !).
 #
-# To update the man pages in tree before a release, you need to
-# ensure you have the 'click_man' package installed, and run:
+# To update the man pages in tree before a release, run:
 #
-# python3 setup.py --command-packages=click_man.commands man_pages
+#     tox -e man
 #
 # Then commit the result.
 #