add more documentation about rendering options

[vincentsarago]

colormaps: https://cogeotiff.github.io/rio-tiler/colormap/
rescaling
color_formula
...

[vincentsarago]

some nice examples: https://docs.charter.uat.esaportal.eu/webPortal/geobrowser/titiler/

[dchirst]

I'll start looking at this one next (since I can't assign myself to a task).

[vincentsarago]

done in #620 🙏