Integrate Issue and PR templates

[mfranzke]

We'd like to mention aspects like e.g. our CONTRIBUTION guidelines and aspects someone should check before UI contributions like e.g. emulating forced-colors, prefers-colors-scheme (probably we mention those in an extra document, that we even also link from the CONTRIBUTION file.

https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository