docs: add CEL filter example for non-code changes

Add comprehensive example showing how to use CEL expressions to filter
PipelineRuns and exclude changes that only affect documentation,
configuration files, or metadata files. This addresses user feedback
about needing precise examples for crafting CEL regex expressions.

The example demonstrates the !files.all.all() syntax to exclude PRs
when all changes match patterns for docs/, .md files, and common
metadata files like .gitignore, OWNERS, PROJECT, and LICENSE.

Fixes: SRVKP-4602

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Chmouel Boudjnah <chmouel@redhat.com>

Author: chmouel

docs/content/docs/guide/matchingevents.md

@@ -375,6 +375,27 @@ This example will match modified files with the name of test.go:
       files.modified.exists(x, x.matches('test.go'))
 ```
 
+### Filtering PipelineRuns to exclude non-code changes
+
+This example demonstrates how to filter `pull_request` events to exclude changes that only affect documentation, configuration files, or other non-code files. This is useful when you want to run tests only when actual code changes occur:
+
+```yaml
+pipelinesascode.tekton.dev/on-cel-expression: |
+  event == "pull_request"
+  && target_branch == "main"
+  && !files.all.all(x, x.matches('^docs/|\\.md$|^(?:.*/)?(\.gitignore|OWNERS|PROJECT|LICENSE)$'))
+```
+
+This expression will:
+
+* Only match `pull_request` events targeting the `main` branch
+* **Exclude** the PipelineRun if all changed files match any of the following patterns:
+  * Files in the `docs/` directory (`^docs/`)
+  * Markdown files (`.md$`)
+  * Common repository metadata files (`.gitignore`, `OWNERS`, `PROJECT`, `LICENSE`)
+
+The `!files.all.all(x, x.matches('pattern'))` syntax means "not all files match the pattern", which effectively means "trigger only if at least one file doesn't match the exclusion pattern" (i.e., there are meaningful code changes).
+
 ### Matching PipelineRun to an event (commit, pull_request) title
 
 This example will match all pull requests starting with the title `[DOWNSTREAM]`: