Add merge queue trigger for required workflows (#6860)

**Context:**
This pull request adds a new trigger to the existing workflows that run
`on.pull_request`. This trigger indicates to GitHub which workflows
needs to be run when a merge queue is building.

**Description of the Change:**
The change is adding `on.merge_group` to our required workflows.

**Benefits:**
This change itself will not enable merge queues, that needs to be
enabled from the admin settings of branch protection rules.

The changes in this PR mainly tell merge queues which workflows to run.

**Possible Drawbacks:**
Usage of merge queue is a new thing for pennylane, if issues arise we
can rollback.

**Related GitHub Issues:**
None. [sc-82039]

.github/workflows/docs.yml

@@ -16,6 +16,9 @@
 
 name: "Documentation check"
 on:
+  merge_group:
+    types:
+      - checks_requested
   pull_request:
     types:
       - opened

.github/workflows/format.yml

@@ -1,5 +1,8 @@
 name: Formatting check
 on:
+  merge_group:
+    types:
+      - checks_requested
   pull_request:
     types:
       - opened

.github/workflows/module-validation.yml

@@ -1,6 +1,9 @@
 name: Validate module imports
 
 on:
+  merge_group:
+    types:
+      - checks_requested
   pull_request:
     types:
       - opened

.github/workflows/tests-gpu.yml

@@ -3,6 +3,9 @@ on:
   push:
     branches:
       - master
+  merge_group:
+    types:
+      - checks_requested
   pull_request:
     types:
       - opened

.github/workflows/tests.yml

@@ -14,6 +14,9 @@ on:
   # Scheduled trigger on Monday at 2:47am UTC
   schedule:
     - cron: "47 2 * * 1"
+  merge_group:
+    types:
+      - checks_requested
 
 concurrency:
   group: unit-tests-${{ github.ref }}