Fix content links/ Add additional Thread section

[Ihavesoul]

Hello everyone. I really respect this book, and really want to read it in a convenient format. The content broke recently, I don't know what it is connected with, but this PR should fix this problem. I hope that the author will not mind that I removed the section about AppDomains (because there is no more information about them) and replaced it with a section with Thread. In any case, I will be glad if you point out the comments