Revise pull request template (#18744)

* Revise pull request template

* Reference compiled docs in PR template

Refer readers to https://k8s.io/contribute/start/

This keeps the template short, and it lets Hugo use templating for
the current version.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,17 +1,20 @@
->^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-> Remember to delete this note before submitting your pull request.
->
-> For pull requests on 1.18 Features: set Milestone to 1.18 and Base Branch to dev-1.18
->
-> For pull requests on Chinese localization, set Base Branch to release-1.16
-> Feel free to ask questions in #kubernetes-docs-zh
->
-> For pull requests on Korean Localization: set Base Branch to dev-1.16-ko.\<latest team milestone>
->
-> If you need Help on editing and submitting pull requests, visit:
-> https://kubernetes.io/docs/contribute/start/#improve-existing-content.
->
-> If you need Help on choosing which branch to use, visit:
-> https://kubernetes.io/docs/contribute/start#choose-which-git-branch-to-use.
->^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
->
+<!-- 🛈
+
+ Hello!
+
+ Remember to ADD A DESCRIPTION and delete this note before submitting
+ your pull request. The description should explain what will change,
+ and why.
+
+ For overall help on editing and submitting pull requests, visit:
+  https://kubernetes.io/docs/contribute/start/#improve-existing-content
+
+ Use the default base branch, “master”, if you're documenting existing
+ features in the English localization.
+
+ If you're working on a different localization (not English), or you
+ are documenting a feature that will be part of a future release, see
+ https://kubernetes.io/docs/contribute/start#choose-which-git-branch-to-use
+ for advice.
+
+-->