Document common hook points for build extensions

We have https://learn.microsoft.com/visualstudio/msbuild/msbuild-targets#sdk-and-default-build-targets but it's not super clear, I think it'd be helpful to have more clear documentation for common use cases.

(https://mastodon.social/@mcc/112451423829765088)

Some examples

| Scenario | Hook point |
|--------|--------|
| "I just need to do something early in the build" | `BeforeTargets="BeforeBuild"` |
| "I need to modify the list of resolved references before anything uses it" | `AfterTargets="ResolveReferences"` |
| "I want to generate some C# source" | `BeforeTargets="BeforeCompile"` |
| "I have some files that I would want to add to `Content` or `None` to get copied to the output, but I generate them during the build" | `BeforeTargets="AssignTargetPaths"` |
| "I need to produce or change an output before this project is done" | `AfterTargets="AfterBuild"` |