Integrate DWARF plugin into SCTK release and correct documentation

We evidently have a plugin to collect certain information related to DWARF references. See, e.g., the [SCTK RTD](https://scancode-toolkit.readthedocs.io/en/latest/plugins/dwarf.html). I tested these instructions yesterday using 21.2.9, with this result: `Error: no such option: --dwarf`.

It appears that the plugin exists only in a branch and is not packaged in PyPI. See, e.g.,

https://github.com/nexB/scancode-plugins/tree/develop/binary-analysis/scancode-compiledcode

https://github.com/nexB/scancode-plugins/tree/develop/binary-analysis

It would be useful to be able to test it in a release (including whether it works with Python 3), and to update the RTD documentation to provide a complete and accurate description of how to run the plugin so it can be tested (and, if it works as desired, used).