Fix torrent magnet links, add a helper link for magnet URLs.

[dessalines]

Pre-Flight checklist

• Did you check to see if this issue already exists?
• This is a single feature request. (Do not put multiple feature requests in one issue)
• This is not a question or discussion. (Use https://lemmy.ml/c/jerboa for that)

Describe The Feature Request Below

I checked again recently, and magnet links don't seem to be working correctly atm.

I've added a helper link to lemmy's docs here, that I'll try to figure out how to link to.