Update accessibility contribution guidance in bug report and PR templates (#7142)

Co-authored-by: Scott Cranfill <scott@scottcranfill.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,5 +5,7 @@ Before submitting, please review the contributor guidelines <https://docs.wagtai
 * Do the tests still pass? (https://docs.wagtail.io/en/latest/contributing/developing.html#testing)
 * Does the code comply with the style guide? (Run `make lint` from the Wagtail root)
 * For Python changes: Have you added tests to cover the new/fixed behaviour?
-* For front-end changes: Did you test on all of Wagtail’s supported browsers? **Please list the exact versions you tested**.
+* For front-end changes: Did you test on all of [Wagtail’s supported environments](https://docs.wagtail.io/en/latest/contributing/developing.html#browser-and-device-support)?
+    * **Please list the exact browser and operating system versions you tested**.
+    * **Please list which assistive technologies you tested**.
 * For new features: Has the documentation been updated accordingly?

docs/contributing/html_guidelines.rst

@@ -13,5 +13,6 @@ Principles
 
 * Write `valid HTML <https://validator.w3.org/nu/>`_. We target the HTML5 doctype.
 * Write `semantic HTML <https://html5doctor.com/element-index/>`_.
+* Consult `ARIA Authoring Practices <https://w3c.github.io/aria-practices/>`_, in particular `No ARIA is better than Bad ARIA <https://w3c.github.io/aria-practices/#no_aria_better_bad_aria>`_.
 * Attach JavaScript behavior with ``data-`` attributes, rather than classes or IDs.
 * For comments, use Django templates syntax instead of HTML.