Add Documentation PR section to the issue template (kubernetes#2016)

* Add Documentation PR section to the issue template

* Add alpha, beta, stable under documentation PR to the issue template

Co-authored-by: Anna Jung <[email protected]>

Author: annajung

ISSUE_TEMPLATE.md

@@ -8,5 +8,9 @@
   - Alpha release target (x.y)
   - Beta release target (x.y)
   - Stable release target (x.y)
+- Documentation PR: <!-- link to kubernetes/website PR -->
+  - Alpha:
+  - Beta:
+  - Stable:
 
 _Please keep this description up to date. This will help the Enhancement Team to track the evolution of the enhancement efficiently._