Add GitHub Issue template

[felker]

See #596 (comment)

If we are encouraging readers to report issues with the documentation source directly on GitHub, we should consider adding a minimalist template so that it is less daunting to users who are unfamiliar with the process / promote the quality of the reports
https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository