chore(ci): allowlist devnets/bal/4 for docs publishing

Adds the `bal-devnet-4` label to the site's version switcher and to
the `docs-build` dispatch dropdown so pushes and manual dispatches to
`devnets/bal/4` publish at `/docs/devnets/bal/4/`.

Author: danceratopz

.github/configs/docs-branches.yaml

@@ -12,3 +12,5 @@ default: "forks/amsterdam"
 branches:
   - path: "forks/amsterdam"
     label: "Amsterdam"
+  - path: "devnets/bal/4"
+    label: "bal-devnet-4"

.github/workflows/docs-build.yaml

@@ -46,6 +46,7 @@ on:
         type: choice
         options:
           - forks/amsterdam
+          - devnets/bal/4
         default: forks/amsterdam
       ref:
         description: "Ref to build (branch, tag, or commit SHA). Empty = branch tip."