Add support for multiple languages in the documentation

[fizruk]

Work in progress. As it turns out, multiple languages and versioning are hard to make work with each other.

• Reuse rzk/ChangeLog.md and CONTRIBUTORS.md, instead of duplicating it.
• Add link to default language
  • Will be done manually on gh-pages when a corresponding release is ready.
• Figure out correct link to Playground (solved with a redirect page)
• Check Rzk files for all languages
• Stay on page when switching language? (this is currently a feature only available to sponsors of MkDocs Material)
  • Will consider later.

You can check out result of this PR:

• English at http://rzk-lang.github.io/rzk/en/
• Russian at http://rzk-lang.github.io/rzk/ru/
  • Home
  • Getting started (including a new section on Dependent Types)
  • Reference (will update and translate in a separate PR)

Additional changes:

• Add (default) social cards
• Integrate ToC on the left
• Use Inria Sans for English, PT Sans for Russian