Add sphinx-argparse to generate usage

Move parser generation into a seperate file to avoid import issues with bad python modules during docs gen.
With the requirements.txt provided to readthedocs, there shouldn't need to be any other changes?
Also I've never been able to run `make test` so I'm not actually sure if the changes break sby, but they shouldn't.

docs/source/conf.py

@@ -39,3 +39,4 @@ html_theme_options = {
 }
 
 extensions = ['sphinx.ext.autosectionlabel']
+extensions += ['sphinxarg.ext']

docs/source/index.rst

@@ -16,6 +16,7 @@ formal tasks:
 
    install.rst
    quickstart.rst
+   usage.rst
    reference.rst
    autotune.rst
    verilog.rst

docs/source/requirements.txt

@@ -1 +1,2 @@
 furo
+sphinx-argparse

docs/source/usage.rst

@@ -0,0 +1,11 @@
+Using `sby`
+===========
+
+Once SBY is installed and available on the command line as `sby`, either built from source or using
+one of the available CAD suites, it can be called as follows.  Note that this information is also
+available via `sby --help`.  For more information on installation, see :ref:`install-doc`.
+
+.. argparse::
+    :filename: ../sbysrc/sby_cmdline.py
+    :func: parser_func
+    :prog: sby