docs: add location prose

docs/api/location.md

@@ -1,3 +1,18 @@
+Locations can be used in TITAN to differentiate agents by "geography".  The primary features of locations are:
+
+* Differentiated parameters via location scaling/overrides
+    - Allows different demographics or interventions by location
+    - See [params app for details](https://pph-collective.github.io/titan-params-app/#/params#location-1)
+* Location based assorting (including based on neighboring locations)
+    - Can have agents assort with agents from their own location vs neighbors vs all others
+    - Neighboring locations are determined by the `edges` defined in `params.location`
+    - See [params app for details on assorting rules](https://pph-collective.github.io/titan-params-app/#/params#assort_mix-1)
+    - It is also possible to define edges via a geography CSV (see [utils](/api/utilities/#titan.utils.grid_file_to_edges)). This is also exposed via the `grid2edges` command line utility (run `grid2edges --help` for usage).
+* Migration between locations
+    - When an agent migrates locations, they adopt the parameters of their new location
+    - Migration can cause the population numbers in a location to drift over time
+    - See [params app for details](https://pph-collective.github.io/titan-params-app/#/params#location-1)
+
 ## Location
 
 ::: titan.location.Location