docs: simplify PR template documentation

The issue templates are essentially more advanced
versions of the ones that were being maintained in
the `templates` directory.

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,7 +6,7 @@
 
 #### Prerequisites checklist
 
-- [ ] I have read the [contributing guidelines](https://github.com/eslint/eslint/blob/HEAD/CONTRIBUTING.md).
+- [ ] I have read the [contributing guidelines](../CONTRIBUTING.md).
 
 #### What is the purpose of this pull request? (put an "X" next to an item)
 
@@ -16,16 +16,16 @@
 -->
 
 [ ] Documentation update
-[ ] Bug fix ([template](https://raw.githubusercontent.com/eslint/eslint/HEAD/templates/bug-report.md))
-[ ] New rule ([template](https://raw.githubusercontent.com/eslint/eslint/HEAD/templates/rule-proposal.md))
-[ ] Changes an existing rule ([template](https://raw.githubusercontent.com/eslint/eslint/HEAD/templates/rule-change-proposal.md))
+[ ] Bug fix ([template](ISSUE_TEMPLATE/bug-report.yml))
+[ ] New rule ([template](ISSUE_TEMPLATE/new-rule.yml))
+[ ] Changes an existing rule ([template](ISSUE_TEMPLATE/rule-change.yml))
 [ ] Add autofix to a rule
 [ ] Add a CLI option
 [ ] Add something to the core
 [ ] Other, please explain:
 
 <!--
-    If the item you've checked above has a template, please paste the template questions below and answer them. (If this pull request is addressing an issue, you can just paste a link to the issue here instead.)
+    If this pull request is addressing an issue, you can just paste a link to the issue here. If there is no corresponding issue, but the item you've checked above has a template, you can either file an issue according to that template or copy and paste the template questions below and answer them.
 -->
 
 <!--