Design docs

[gavv]

• Configure mkdocs (Add mkdocs #18, docs; nix-shell script; gitignore updated #19)
• Overview
  • Preface (docs; nix-shell script; gitignore updated #19, Restructure & refine initial docs #22)
  • Features (Restructure & refine initial docs #22)
  • Platforms (docs; nix-shell script; gitignore updated #19, Restructure & refine initial docs #22)
  • Use cases
  • Applications (Restructure & refine initial docs #22)
• Implementation
  • Architecture
    • Components
    • Entities
    • Modules
  • Use of Roc Toolkit (docs; nix-shell script; gitignore updated #19, Restructure & refine initial docs #22)
  • Relation to Roc Cast (docs; nix-shell script; gitignore updated #19, Restructure & refine initial docs #22)