Redirect /upgrade to /doc/upgrade#manual

- We've had 2 places to document manual upgrades since 2016,
  /doc/upgrade then pointing to /upgrade for the actual meat. Having a
  single place prevents people to jump around needlessly.

- Tails Upgrader has been pointing to /doc/upgrade since 2014 and these
  old versions of Tails will continue to point to it.

- We could as well redirect /doc/upgrade/#manual to /upgrade but
  /doc/upgrade has the advantage of putting both techniques in
  perspective.

- /doc/upgrade (formerly /doc/first_steps/upgrade) is very well indexed
  by search engines while we prevented /upgrade to be indexed by
  'noindex'.