Our docs had a bunch of problems with links. Many things have moved to

HTTPS or moved around and have permanent redirects. Some things just

don't exist anymore. This change fixes up almost everything in there. I

left a few things where there's a redirect where the old URL is still

the ideal one to list. There are also still a couple of links to

integrations pages on reviewboard.org that don't exist (but that needs

to be fixed on the website side).

Ran make linkcheck and saw that (almost) everything is good now.