_content/doc/go1.20: add link to PGO user guide

For golang/go#55022.

Change-Id: Ib2420c000dd0d6e7e272e217473e4ec192d4c730
Reviewed-on: https://go-review.googlesource.com/c/website/+/464477
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/doc/go1.20.html

@@ -323,6 +323,7 @@ <h2 id="compiler">Compiler</h2>
   Go 1.20 uses PGO to more aggressively inline functions at hot call sites.
   Benchmarks for a representative set of Go programs show enabling
   profile-guided inlining optimization improves performance about 3–4%.
+  See the <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
   We plan to add more profile-guided optimizations in future releases.
   Note that profile-guided optimization is a preview, so please use it
   with appropriate caution.