Fix linkcheck.py

[XiangRongLin]

While trying to migrate to github actions from travis CI I noticed that the script finishes with errors. Part of them are correct, but part of them probably come from travis CI being deactivated for quite some while, meaning the erros just got silently merged it.

Types I could identify:

1. ~~Link to other files like ../07_maintainers_view#merge-changes-from-weblate-into-newpipe.~~ DONE
2. ~~Invalid links due to wording changes in title, but not link.~~ DONE
3. ~~requests.get does not have a timeout. One website does not return anything => script waits endlessly~~ DONE
4. Figure out why 06_releasing.md: Could not find target for #creating_a_new_release is getting thrown as error even though a title like this exists
5. Find a proper solution for title with - aka F-Droid, see how they are compared

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~~I'm currently struggeling to get it to run locally~~. I may not find the time to do this myself

I fixed what I could do quickly in #32. Feel free to hijack that PR or open a seperate one without my hacky fixes.