Clarify changelog preview workflow

Give the Towncrier validation workflow a descriptive name and explain that its draft render does not modify the repository.

Author: eric-heiden

.github/workflows/changelog.yml .github/workflows/changelog-preview.yml.github/workflows/changelog.yml renamed to .github/workflows/changelog-preview.yml

@@ -1,4 +1,5 @@
-name: Changelog
+# Verify that Towncrier can render pending fragments without changing the repository.
+name: Changelog fragment preview
 permissions:
   contents: read