docs: document smart hook file classification in CONTRIBUTING (#23370)

The git hooks now classify staged files and select either the full
or lightweight make target. This was missing from the contributing
guide after #23358 landed.

Also add actionlint config to suppress a pre-existing SC2016 false
positive in the triage workflow. Shellcheck disable directives
don't work inside heredocs when actionlint drives shellcheck.

.github/actionlint.yaml

@@ -0,0 +1,9 @@
+paths:
+  # The triage workflow uses a quoted heredoc (<<'EOF') with ${VAR}
+  # placeholders that envsubst expands later. Shellcheck's SC2016
+  # warns about unexpanded variables in single-quoted strings, but
+  # the non-expansion is intentional here. Actionlint doesn't honor
+  # inline shellcheck disable directives inside heredocs.
+  .github/workflows/triage-via-chat-api.yaml:
+    ignore:
+      - 'SC2016'