Add direct issue creation to 404 page

[Confectrician]

Idea:

Add a link to our 404 page, which has a dynamically generated url to create an issue in this repository based on https://docs.github.com/en/issues/tracking-your-work-with-issues/creating-an-issue#creating-an-issue-from-a-url-query

With this we can give users a (relatively) easy possibility to report broken links. The only thing needed would be a github acocunt, which may limit the entrs limit a bit.