[CI/Build] Adopt Mergify for auto-labeling PRs (#9259)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

.github/mergify.yml

+pull_request_rules:
+- name: label-documentation
+  description: Automatically apply documentation label
+  conditions:
+    - or:
+      - files~=^[^/]+\.md$
+      - files~=^docs/
+  actions:
+    label:
+      add:
+        - documentation
+
+- name: label-ci-build
+  description: Automatically apply ci/build label
+  conditions:
+    - files~=^\.github/
+    - files~=\.buildkite/
+    - files~=^cmake/
+    - files=CMakeLists.txt
+    - files~=^Dockerfile
+    - files~=^requirements.*\.txt
+    - files=setup.py
+  actions:
+    label:
+      add:
+        - ci/build
+
+- name: label-frontend
+  description: Automatically apply frontend label
+  conditions:
+    - files~=^vllm/entrypoints/
+  actions:
+    label:
+      add:
+        - frontend
+
+- 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