Prettify/populate sphinx-doc

[dweindl]

https://petab-select.readthedocs.io/en/develop/ still needs some work

• nicer landing page
• petab-select input files File formats to RTD #102
• link to pypesto integration example
• link to other tools supporting petab-select

[dweindl]

@dilpath : do you prefer having File formats and Test cases only on readthedocs, or README+readthedocs?

[dilpath]

I would prefer having it on readthedocs only