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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.

Otherwise the logo does not work on systems without exo font installed
(font is replaced)