Update PR template for 1.13 release (#10603)

* Update PR template for 1.13 release

Also updates links to documentation for submitting PRs

* Update contributing docs for new branch strategy

Branches for upcoming releases are now dev-X.Y instead of
release-X.Y

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,12 @@
 >^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 > Please delete this note before submitting the pull request.
-> For 1.12 Features: set Milestone to 1.12 and Base Branch to release-1.12
-> Help editing and submitting pull requests:  https://deploy-preview-9510--kubernetes-io-master-staging.netlify.com/docs/contribute/start/#submit-a-pull-request.
-> Help choosing which branch to use, see
+>
+> For 1.13 Features: set Milestone to 1.13 and Base Branch to dev-1.13
+>
+> Help editing and submitting pull requests:
+> https://kubernetes.io/docs/contribute/start/#improve-existing-content.
+>
+> Help choosing which branch to use:
 > https://kubernetes.io/docs/contribute/start#choose-which-git-branch-to-use.
 >^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 >

content/en/docs/contribute/start.md

@@ -179,7 +179,7 @@ to base your work on. Use these guidelines to make the decision:
 
 - Use `master` for fixing problems in content that is already published, or
   making improvements to content that already exists.
-- Use a release branch (such as `release-1.12`) to document upcoming features
+  - Use a release branch (such as `dev-{{< release-branch >}}` for the {{< release-branch >}} release) to document upcoming features
   or changes for an upcoming release that is not yet published.
 - Use a feature branch that has been agreed upon by SIG Docs to collaborate on
   big improvements or changes to the existing documentation, including content