define line and area on mapbox [JS]

[Mahdis-z]

closes #1412

[nicolaskruchten]

• section/example headers need to be descriptive of the examples: less emphasis on tiles and tokens outside of the "layers" page
• more explanatory text around "there are N ways to do this, here is how they compare, here are links to the other ways, here is one example of each"
• Line and area should be separate pages
• While we're at it we likely need "filled areas on maps" (non-mapbox)
• Line: we should document the "broken line" trick
• Area: we should document the "broken trace" trick and we should point out that there are basically three ways in Mapbox to make areas... fill-to-self, geojson layers, and the new choroplethmapbox trace type
• for the non-mapbox case there is only the one way to make filled areas today.

note: let's start with the Python versions of these pages please

[nicolaskruchten]

Please rebase onto s-d-m- to get CI up and running :)

[nicolaskruchten]

Should match the permalinks and contents of plotly/plotly.py-docs#125