Document guidelines for using EF Core with trimming or NativeAOT

[roji]

Depending on where we end up, this may just be "disable it", or a set of coding guidelines to make things work well when trimming is enabled.

[AndriySvyryd]

Using a compiled model with precompiled queries is recommended for trimming and required for NativeAOT.

Also update https://aka.ms/efcore-docs-trimming link