Added link to EF Core spatial doc page

(cherry picked from commit a1f05ca)

Author: roji

doc/mapping/nts.md

@@ -1,5 +1,8 @@
 ## Spatial Mapping with NetTopologySuite
 
+> [!NOTE]
+> It's recommended that you start by reading the general [Entity Framework Core docs on spatial support](https://docs.microsoft.com/en-us/ef/core/modeling/spatial).
+
 PostgreSQL supports spatial data and operations via [the PostGIS extension](https://postgis.net/), which is a mature and feature-rich database spatial implementation. .NET doesn't provide a standard spatial library, but [NetTopologySuite](https://github.com/NetTopologySuite/NetTopologySuite) is quite a good candidate. The Npgsql EF Core provider has a plugin which allows you to map NetTopologySuite's types PostGIS columns, and even translate many useful spatial operations to SQL. This is the recommended way to interact with spatial types in Npgsql.
 
 Note that the EF Core NetTopologySuite plugin depends on [the Npgsql ADO.NET NetTopology plugin](http://www.npgsql.org/doc/types/nts.html), which provides NetTopologySuite support at the lower level. The EF Core plugin automatically arranged for the ADO.NET plugin to be set up.