[WIP] Introduce new about/website page (en)

@hsbt, @chikamichi, @postmodern, @makimoto, @sorah, @zzak

This commit moves most of the contents of the credits footer to a dedicated page, `en/about/website`.
IMO the footer got too crowded by now, and a separate page could contain additional information.

Please comment on:
- what must stay in the footer
- what content is still missing on about/website
- do we need to add more legal information (license), and where?

Currently proposed, highly stripped-down footer:
> `<a href="/en/about/website/">This website</a> is proudly maintained
      by members of the Ruby community.`

I don't want to merge before we have a in some degree "final" version.