Add TW coverage issue template

Following gitlab-com/www-gitlab-com!103608 (merged), let's have our own coverage template 🙂

Borrowed from:

• https://gitlab.com/gitlab-org/gitlab-design/-/blob/master/.gitlab/issue_templates/UX%20Coverage.md
• https://gitlab.com/gitlab-com/Product/-/blob/main/.gitlab/issue_templates/PM-Coverage.md