docs/shell.rst: Auto-generate shell command reference

We use a slightly modified version of sphinx-argparse
(with patch https://github.com/alex-rudakov/sphinx-argparse/pull/136 applied)
in order to generate the command reference for each shell command in the
manual.

As the upstream repository seems unmaintained for ~2 years, let's use
the osmocom 'fork' with that above-mentioned patch applied.

Change-Id: I134f267cf53c7ecbc8cbfb33a8766d63bf4a8582
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index efb913e..60061e3 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -21,6 +21,7 @@
 # attempt to build documentation
 pip install sphinx
 pip install sphinxcontrib-napoleon
+pip3 install -e 'git+https://github.com/osmocom/sphinx-argparse@master#egg=sphinx-argparse'
 (cd docs && make html)
 
 # run the test with physical cards