Update issue templates with a new "Who can work on this" section

.github/ISSUE_TEMPLATE/BUG_REPORT.md

@@ -32,3 +32,13 @@ Any other relevant information. For example, why do you consider this a bug and
 -   Django version: Look in your requirements.txt, or run `pip show django | grep Version`.
 -   Wagtail version: Look at the bottom of the Settings menu in the Wagtail admin, or run `pip show wagtail | grep Version:`.
 -   Browser version: You can use https://www.whatsmybrowser.org/ to find this out.
+
+### Working on this
+
+<!--
+  Do you have thoughts on skills needed?
+  Are you keen to work on this yourself once the issue has been accepted?
+  Please let us know here.
+-->
+
+Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.

.github/ISSUE_TEMPLATE/DOCUMENTATION.md

@@ -29,3 +29,13 @@ assignees: ''
 <!--
   If you're suggesting a very specific change to the documentation, feel free to directly submit a pull request.
 -->
+
+### Working on this
+
+<!--
+  Do you have thoughts on skills needed?
+  Are you keen to work on this yourself once the issue has been accepted?
+  Please let us know here.
+-->
+
+Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.

.github/ISSUE_TEMPLATE/FEATURE_REQUEST.md

@@ -39,3 +39,13 @@ assignees: ''
 -->
 
 (Write your answer here.)
+
+### Working on this
+
+<!--
+  Do you have thoughts on skills needed?
+  Are you keen to work on this yourself once the issue has been accepted?
+  Please let us know here.
+-->
+
+Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.