ci: introduce Mergify bot to notify on PR conflicts (#4043)

This commit introduces Mergify, a powerful bot designed to assist with
automated merging and other CI-related tasks. As an initial step, we
enable a basic feature: automatically notifying users when a pull
request has merge conflicts.

When a conflict is detected, Mergify will add a label to the PR. This
label will be removed once the conflict is resolved.
This is foundation PR to activate the bot and start using it for
backports too.

In the future, we plan to expand Mergify’s role to include auto-merging,
as discussed in #1667, once the project is ready.

Signed-off-by: Sébastien Han <seb@redhat.com>

.github/mergify.yml

@@ -0,0 +1,23 @@
+pull_request_rules:
+- name: ping author on conflicts and add 'needs-rebase' label
+  conditions:
+      - conflict
+      - -closed
+  actions:
+    label:
+      add:
+        - needs-rebase
+    comment:
+      message: >
+       This pull request has merge conflicts that must be resolved before it
+       can be merged. @{{author}} please rebase it.
+       https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork
+
+- name: remove 'needs-rebase' label when conflict is resolved
+  conditions:
+      - -conflict
+      - -closed
+  actions:
+    label:
+      remove:
+        - needs-rebase