Clever redirects

Instead of relying on https://docs.gitlab.com/ee/development/documentation/#changing-document-location to create the redirect pages, we should use https://github.com/gjtorikian/nanoc-redirector.

That's more efficient and works like Jekyll's redirect plugin.

If we implement this, we'll be able to retain just the new page and remove the old ones. Right now, we're compelled to keep the old pages, so there are dozens of docs that say "this doc was moved to another location".

Limitation

There's a limitation though, this only works with clean and absolute URLs, since the plugin adds an index.html. So, we can use:

---
redirect_from:
  - /ee/api/
---

but we cannot use relative links and markdown:

---
redirect_from:
  - ../api/README.md
---

Proposal

Contribute to upstream so that we can also use relative URLs! I've opened an issue, but not sure this will be picked up https://github.com/gjtorikian/nanoc-redirector/issues/11.