Fix links in docs to the kenreitz.org domain, which expired.

[mila]

When I was studying Contributor’s Guide, I found that links to kenreitz.org are broken.

• "The Future of Python HTTP" was archived by Internet Archive, so I linked there.
• "Be cordial or be on your way" was not archived, so I removed the link.
• "A Kenneth Reitz Project." in a footer is broken too, but I did not touch that link.