docs: Update the custom_script default and documentation

* The "startGroup GroupTitle"/"endGroup" commands allow one to create
  timed foldable sections in the GitHub action logging.

README.md

@@ -52,12 +52,20 @@ Among `"minimal"`, `"4.07-flambda"`, `"4.09-flambda"`.
 
 *Optional* The main script run in the container; may be overridden. Default:
 
-    opam config list; opam repo list; opam list
-    opam pin add -n -y -k path $PACKAGE .
-    opam update -y
-    opam install -y -v -j 2 $PACKAGE
-    opam list
-    opam remove $PACKAGE
+    startGroup Print opam config
+      opam config list; opam repo list; opam list
+    endGroup
+    startGroup Fetch dependencies
+      opam pin add -n -y -k path $PACKAGE .
+      opam update -y
+    endGroup
+    startGroup Build
+      opam install -y -v -j 2 $PACKAGE
+      opam list
+    endGroup
+    startGroup Uninstallation test
+      opam remove $PACKAGE
+    endGroup
 
 *Note: this option is named `custom-script` rather than `script` or
 `run` to discourage changing its recommended, default value, while

action.yml

@@ -14,12 +14,20 @@ inputs:
   custom_script:
     description: 'The main script run in the container; may be overridden.'
     default: |
-      opam config list; opam repo list; opam list
-      opam pin add -n -y -k path $PACKAGE .
-      opam update -y
-      opam install -y -v -j 2 $PACKAGE
-      opam list
-      opam remove $PACKAGE
+      startGroup Print opam config
+        opam config list; opam repo list; opam list
+      endGroup
+      startGroup Fetch dependencies
+        opam pin add -n -y -k path $PACKAGE .
+        opam update -y
+      endGroup
+      startGroup Build
+        opam install -y -v -j 2 $PACKAGE
+        opam list
+      endGroup
+      startGroup Uninstallation test
+        opam remove $PACKAGE
+      endGroup
 runs:
   using: 'docker'
   image: 'Dockerfile'