[docs] Support anchor links

[brenns10]

I was excited to see the new Docs plugin, but when I downloaded the Python 3 docset, I saw that most of the documentation items were missing, which was unfortunate. I read the code and saw that the items which were missing from the index were items that had anchor links. The code comments say that they are skipped because the web browser won't open them.

I came up with what I think is a clever workaround: create a temporary HTML file which contains a meta directive that tells the browser to immediately redirect to the target page, with the anchor appended. Then, we can use openUrl() on that temporary file without needing to pass an anchor in.

But full disclosure: I have very little C++ under my belt, and no Qt experience. The implementation I made is likely pretty garbage. It would need some love:

1. It doesn't use a proper temporary file, since that would need to be cleaned up later and I didn't know how to handle that.
2. It doesn't properly handle filenames with quotes: they would need to be escaped in order to create valid HTML.

[ManuelSchneid3r]

This issue was not intentionally but automatically closed because I renamed the master branch to main. If you are still interested in getting it merged please reopen against main.