Add Getting Started page to mdocs

[CloudNiner]

This task is to rewrite our GeoTrellis Getting Started page in the new mdocs.

It should include:

• [ ] A short list of key GeoTrellis features and packages (non-exhaustive)
• [ ] A link to our up to date SBT template (if it exists and is up to date)
• [ ] A tutorial based on landsat that reads a raster via RasterSource, selects and combines bands, computes a map algebra operation, and renders the result to PNG

Before starting this task, we'll want to review with @pomadchin the above list to ensure we've captured common getting started questions he's fielded on Gitter.

[pomadchin]

@CloudNiner looks good to me. I would add that an example can mimic the landsat tutorial project in some sense. It is popular because it demonstrates not only how to read TIFFs into memory but also how to combine bands, perform a simple map algebra, and render the result.

[CloudNiner]

@pomadchin suggests a Quickstart page as part of this section that walks a user through setting up GT dependency in SBT, then working through the following example:

raster read, reproject, bands selection, computing ndvi and generating png

👍