Add documentation on loc graph approaches

[djgroen]

Essentially, there are three ways we can represent locations in Flee:

• location-based (as we do it)
• region-based (as Alireza has done a few times)
• sparse grid-based, where each area is a square with x km2 and where one only includes squares that meet a certain threshold. This approach Will plans to try out.

[djgroen]

This is in the docs now :).