.github: Add mergify configuration (#3026)

.github/mergify.yml

@@ -0,0 +1,28 @@
+defaults:
+  actions:
+    queue:
+      method: squash
+      commit_message_template: |
+        {{ title }} (#{{ number }})
+        
+        {{ body | get_section("## Commit Message body", "") }}   
+
+pull_request_rules:
+  - name: Ask to resolve conflict
+    conditions:
+      - conflict
+      - -draft # Draft PRs are allowed to have conflicts.
+    actions:
+      comment:
+        message: This pull request has merge conflicts. Could you please resolve them @{{author}}? 🙏
+
+  - name: Add to merge queue
+    conditions:
+      # All branch protection rules are implicit: https://docs.mergify.com/conditions/#about-branch-protection
+      - label=send-it
+    actions:
+      queue:
+
+queue_rules:
+  - name: default
+    conditions: []

.github/pull_request_template.md

@@ -19,3 +19,6 @@
 - [ ] I have made corresponding changes to the documentation
 - [ ] I have added tests that prove my fix is effective or that my feature works
 - [ ] A changelog entry has been made in the appropriate crates
+
+<!-- The below text will appear as the commit message body once we squash-merge the PR. -->
+## Commit message body