Add How To Use Rasters section to new mdocs

[CloudNiner]

Add a Raster section that provides brief how-tos for each of these topics:

• [ ] Terminology including CellType/CellSize/NoData, Tile, Raster definitions in GT context
• [ ] Rendering: ColorMap, ColorRamp, Color Correction
• [ ] Histograms
• [ ] Polygonal Summary

These topics are currently addressed in https://github.com/geotrellis/geotrellis-workshop/blob/master/docs/rendering-images.md, https://github.com/geotrellis/geotrellis-workshop/blob/master/docs/histograms.md.

Also, ensure we instruct users to either run the the raster tests via SBT once or unzip the raster tests package if we end up using any of the test rasters in our examples ( see #3178 )

[CloudNiner]

When writing the CellType/NoData sections, skim #1508 for related concerns that can be addressed within the scope of this page.