[Future Work] Improving proptypes description

[xhluca]

At the moment, the proptypes docstring are vaguely described, and are too much inspired from Cytoscape.js (which is unintuitive for Python users). For the next release, this will be an important focus.

[xhluca]

Comment by Chris: https://github.com/plotly/dash-docs/pull/353#issuecomment-453225601

[xhluca]

Could we update the property docstrings to include reference links to the relevant cytoscape docs? i.e. here for stylesheet, then maybe we include a link to http://js.cytoscape.org/#style? similarly with the other properties as well.

I think that folks that visit this "Reference" page will need links to all of these sections, so if we don't include them in the doc strings, maybe we include a bulleted list at the top of the page linking to the relevant parts of the cytoscape docs.

[xhluca]

Fixed in #29