docs: update contributing documentation and PR template (#1411)

Author: jzempel

.github/CONTRIBUTING.md

@@ -65,7 +65,12 @@ commands are available:
   template.
 
 See Garden's [development documentation](/docs/development.md) for package
-implementation requirements.
+implementation requirements. In general, your code will be expected to follow
+documented standards and prevailing conventions. If you feel the need to
+introduce a new or novel pattern, please start a new
+[discussion](https://github.com/zendeskgarden/react-components/discussions) or
+open an [issue](https://github.com/zendeskgarden/react-components/issues) for
+consideration by the core team.
 
 ## Pull Request Workflow
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -19,11 +19,13 @@
 
 ## Checklist
 
-- [ ] :ok_hand: design updates are Garden Designer approved (add the
-      designer as a reviewer)
+<!-- check the items below that will be completed prior to merge.
+     strikethrough any item text that does not apply to this PR. -->
+
+- [ ] :ok_hand: design updates will be Garden Designer approved (add the designer as a reviewer)
 - [ ] :globe_with_meridians: demo is up-to-date (`yarn start`)
 - [ ] :arrow_left: renders as expected with reversed (RTL) direction
 - [ ] :metal: renders as expected with Bedrock CSS (`?bedrock`)
-- [ ] :wheelchair: analyzed via [axe](https://www.deque.com/axe/) and evaluated using VoiceOver
 - [ ] :guardsman: includes new unit tests
-- [ ] :memo: tested in Chrome, Firefox, Safari, Edge, and IE11
+- [ ] :wheelchair: tested for [WCAG 2.1](https://www.w3.org/TR/WCAG21) AA compliance
+- [ ] :memo: tested in Chrome, Firefox, Safari, and Edge