Add img for rst

[georgringer]

Integrate https://hub.docker.com/r/danielsiepmann/t3-sphinx/ for rendering documentation

[josefglatz]

As helper, while writing your extension documentation? I think I could also include it to the clitools. Anything like ct typo3:docrender ? Would that also help?

[georgringer]

Ask the author, imo he is active in the doc team and on slack what the best way would be.
Imo CT is optional cause rendering ist triggered while writing with the doc extensions by Xavier

[tbal]

Relates to #123