Change docs theme

[a-recknagel]

I initially picked the flask theme because I like the minimal look, but there is other good ones out there. The sphinx_rtd_theme in particular is worth a look since we want to include have included text boxes that happen to look very read-the-docs-y (alternatives include changing the tab-box css to look more flasky or accepting the visual discrepancy).

• [x] wait a little to see if people care about this topic
• [x] decide which theme to pick. default choice: sphinx_rtd_theme

[maxfischer2781]

+1 on sphinx_rtd_theme -- IMO it is well suited for technical documentation.