[Aliki] Allow customizing Aliki's footer

[st0012]

Closes #1451

With this in ruby/ruby:

footer_content:
  Ruby:
    Documentation: index.html
    Official Site: https://www.ruby-lang.org/
    Playground: https://ruby.github.io/play-ruby/
  RESOURCES:
    GitHub: https://github.com/ruby/ruby
    Issue Tracker: https://bugs.ruby-lang.org/projects/ruby-master/issues

It'd generate:

[matzbot]

🚀 Preview deployment available at: https://cd4f0f82.rdoc-6cd.pages.dev (commit: b35e6e2)

[kou]

+1