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We already have a similar and much more detailed section in howto.rst. Further, the included run.py usage print will often be out-of-date and manually updating it every time isn't maintainable.

To manage external links such as for Python Modules, we are now using the extension sphinx.ext.extlinks to define all of them in the file doc/external_links.py.

- change example experiment to pyexps/simple_ping.py
- quickstart.rst: remove repeated information already found in README
- README: add note that simbricks-run is only available in docker containers

- add `Error Opening images/output-base/base.raw`
- move `Getting Help` to the beginning and add content

Also adding some labels needed for referencing various parts of the
rest of the documentation.