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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.

fix container, Python version, path to sphinx' conf.py and add all
required packages

- 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.