_content/doc/go1.24: link to in-tree GOCACHEPROG docs

Currently the release notes link to the proposal for GOCACHEPROG,
which isn't ideal and is also slightly outdated. CL 638995 moves the
protocol documentation to its own package that we can link to and that
can't get out of date, so point the release notes at this
documentation.

For golang/go#71032

Change-Id: I8d4db0a0153333b40f9021f030a36184546bfe51
Reviewed-on: https://go-review.googlesource.com/c/website/+/638378
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

_content/doc/go1.24.md

@@ -141,7 +141,7 @@ The `cmd/go` internal binary and test caching mechanism can now be implemented
 by child processes implementing a JSON protocol between the `cmd/go` tool
 and the child process named by the `GOCACHEPROG` environment variable.
 This was previously behind a GOEXPERIMENT.
-For protocol details, see [#59719](/issue/59719).
+For protocol details, see [the documentation](/cmd/go/internal/cacheprog).
 
 ## Runtime {#runtime}