Link platforms migration info from core docs.

These docs overlap. I think the ultimate final state should be to merge whatever's sustainably useful in the migration doc here and delete that doc.

Maybe once we get to "everything but Apple" we can do that merge and add a quick footnote about the state of Apple.

PiperOrigin-RevId: 523388904
Change-Id: I9c34f120e9272f917bb1ab1f4aab37589117136e

site/en/extending/platforms.md

@@ -41,6 +41,9 @@ Bazel supports the following build scenarios regarding platforms:
 *  **Multi-platform builds** - host, execution, and target platforms are all
    different.
 
+Tip: for detailed instructions on migrating your project to platforms, see
+[Migrating to Platforms](/concepts/platforms).
+
 ## Defining constraints and platforms {:#constraints-platforms}
 
 The space of possible choices for platforms is defined by using the