docs: add pull request template

Add .github/PULL_REQUEST_TEMPLATE.md with What/Why sections and a
single CONTRIBUTING.md guidelines checkbox. Kept minimal per
maintainer preference.

Closes: #968

Co-Authored-By: Claude <claude@anthropic.com>
Signed-off-by: smoparth <smoparth@redhat.com>

Author: 2 people

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+# Pull Request Description
+
+## What
+
+<!-- Brief description of the change. -->
+
+## Why
+
+<!-- Link to issue (Closes #NNN) or explain the motivation. -->
+
+- [ ] PR follows [CONTRIBUTING.md](https://github.com/python-wheel-build/fromager/blob/main/CONTRIBUTING.md) guidelines