doc: re-organize golang.org site content

Remove "References" section.
Remove most articles and redirect to blog.golang.org.
Move /ref/spec and /ref/mem to /doc/spec and /doc/mem.
Remove duplicate links from the remaining
"Documents", "The Project", and "Help" pages.
Defer to the wiki for more links and community content.
Update command reference and mention cover tool.
Add "Pop-out" text to the front page.
Pick one of four videos at random to feature on the front page.

Fixes #2547.
Fixes #5561.
Fixes #6321.

R=r, dominik.honnef
CC=golang-dev
https://golang.org/cl/13724043