[Docs] Add developer doc about CI failures (#18782)

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Co-authored-by: Mark McLoughlin <markmc@redhat.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>

docs/contributing/ci-failures.md

+# CI Failures
+
+What should I do when a CI job fails on my PR, but I don't think my PR caused
+the failure?
+
+- Check the dashboard of current CI test failures:  
+  👉 [CI Failures Dashboard](https://github.com/orgs/vllm-project/projects/20)
+
+- If your failure **is already listed**, it's likely unrelated to your PR.  
+  Help fixing it is always welcome!  
+    - Leave comments with links to additional instances of the failure.  
+    - React with a 👍 to signal how many are affected.
+
+- If your failure **is not listed**, you should **file an issue**.
+
+## Filing a CI Test Failure Issue
+
+- **File a bug report:**  
+    👉 [New CI Failure Report](https://github.com/vllm-project/vllm/issues/new?template=450-ci-failure.yml)
+
+- **Use this title format:**
+  
+    ```
+    [CI Failure]: failing-test-job - regex/matching/failing:test
+    ```
+
+- **For the environment field:**
+  
+    ```
+ Still failing on main as of commit abcdef123
+    ```
+
+- **In the description, include failing tests:**
+  
+    ```
+    FAILED failing/test.py:failing_test1 - Failure description  
+     FAILED failing/test.py:failing_test2 - Failure description  
+    https://github.com/orgs/vllm-project/projects/20  
+    https://github.com/vllm-project/vllm/issues/new?template=400-bug-report.yml  
+    FAILED failing/test.py:failing_test3 - Failure description  
+    ```
+
+- **Attach logs** (collapsible section example):
+    <details>
+    <summary>Logs:</summary>
+
+    ```text
+    ERROR 05-20 03:26:38 [dump_input.py:68] Dumping input data  
+    --- Logging error ---  
+    Traceback (most recent call last):  
+      File "/usr/local/lib/python3.12/dist-packages/vllm/v1/engine/core.py", line 203, in execute_model  
+        return self.model_executor.execute_model(scheduler_output)  
+    ...
+    FAILED failing/test.py:failing_test1 - Failure description  
+    FAILED failing/test.py:failing_test2 - Failure description  
+    FAILED failing/test.py:failing_test3 - Failure description  
+    ```
+  
+    </details>
+
+## Logs Wrangling
+
+Download the full log file from Buildkite locally.
+
+Strip timestamps and colorization:
+
+```bash
+# Strip timestamps
+sed -i 's/^\[[0-9]\{4\}-[0-9]\{2\}-[0-9]\{2\}T[0-9]\{2\}:[0-9]\{2\}:[0-9]\{2\}Z\] //' ci.log
+
+# Strip colorization
+sed -i -r 's/\x1B\[[0-9;]*[mK]//g' ci.log
+```
+
+Use a tool for quick copy-pasting:
+
+```bash
+tail -525 ci_build.log | wl-copy
+```
+
+## Investigating a CI Test Failure
+
+1. Go to 👉 [Buildkite main branch](https://buildkite.com/vllm/ci/builds?branch=main)  
+2. Bisect to find the first build that shows the issue.  
+3. Add your findings to the GitHub issue.  
+4. If you find a strong candidate PR, mention it in the issue and ping contributors.
+
+## Reproducing a Failure
+
+CI test failures may be flaky. Use a bash loop to run repeatedly:
+
+```bash
+COUNT=1; while pytest -sv tests/v1/engine/test_engine_core_client.py::test_kv_cache_events[True-tcp]; do  
+  COUNT=$[$COUNT + 1]; echo "RUN NUMBER ${COUNT}";  
+done
+```
+
+## Submitting a PR
+
+If you submit a PR to fix a CI failure:
+
+- Link the PR to the issue:  
+  Add `Closes #12345` to the PR description.
+- Add the `ci-failure` label:  
+  This helps track it in the [CI Failures GitHub Project](https://github.com/orgs/vllm-project/projects/20).
+
+## Other Resources
+
+- 🔍 [Test Reliability on `main`](https://buildkite.com/organizations/vllm/analytics/suites/ci-1/tests?branch=main&order=ASC&sort_by=reliability)
+- 🧪 [Latest Buildkite CI Runs](https://buildkite.com/vllm/ci/builds?branch=main)
+
+## Daily Triage
+
+Use [Buildkite analytics (2-day view)](https://buildkite.com/organizations/vllm/analytics/suites/ci-1/tests?branch=main&period=2days) to:
+
+- Identify recent test failures **on `main`**.
+- Exclude legitimate test failures on PRs.
+- (Optional) Ignore tests with 0% reliability.
+
+Compare to the [CI Failures Dashboard](https://github.com/orgs/vllm-project/projects/20).