[ci] Skip Logstash PR checks when docs change (#15650)

Changes made under `docs/` are unrelated to the PR tests triggered from
this repo[^1]

This commit skips regular PR tests when changes are made (only) under the `docs/` directory.

Note that the current docs-specific Jenkins Job is getting migrated
at an org level[^2] to Buildkite, and this change will be merged soon.

Relates:

- elastic/ingest-dev#2703

[^1]: via `.buildkite/pull_requests_pipeline.yml`
[^2]: https://github.com/elastic/docs/blob/master/.buildkite/pull-requests.org-wide.json

.buildkite/pull-requests.json

@@ -13,7 +13,9 @@
         "always_trigger_comment_regex": "^(?:(?:buildkite\\W+)?(?:build|test)\\W+(?:this|it))",
         "skip_ci_labels": [ ],
         "skip_target_branches": [ ],
-        "skip_ci_on_only_changed": [ ],
+        "skip_ci_on_only_changed": [
+          "^docs/"
+        ],
         "always_require_ci_on_changed": [ ]
       }
     ]