docs(sandboxes): add IPv6 loopback example to Port Forwarding (#2449)

The core IPv6 forwarding fix landed on main via #2552; this carries the
docs example forward into the current Port Forwarding section of
docs/how-it-works/sandboxes/overview.mdx after the page move in #3705.
Remaining scope is documentation only.

Signed-off-by: Andrew White <andrewh@cdw.com>
Co-authored-by: andrewwhitecdw <andrewwhitecdw@users.noreply.github.com>

Author: andrewwhitecdw

docs/how-it-works/sandboxes/overview.mdx

@@ -777,6 +777,14 @@ openshell forward start 8000 my-sandbox -d    # run in background
 ```
 
 OpenShell prints the local URL only after the forward listener is reachable. Background forwards must be tracked locally so `openshell forward list` and `openshell forward stop` can manage them.
+
+To bind the local side to an IPv6 loopback address, pass it positionally as `[bind_address:]port`. IPv6 addresses are bracketed in both the SSH forward argument and the printed access URL:
+
+```shell
+openshell forward start ::1:8000 my-sandbox
+# Local URL: http://[::1]:8000/
+```
+
 CLI forwards run independently of SSH multiplexing settings in your SSH config.
 Foreground forwards end when the command exits; only background forwards appear in `forward list`.