refactor clarity section

[ryanwaits]

feat: revamp the documentation structure: replace the flat alphabetical list of Clarity helpers with a topic-oriented layout (Types, Keywords, Functions → Arithmetics, Sequences, Iterators, Errors, Fungible/Non-Fungible Tokens, …)

• Design new top-level sections
• Types
• Keywords
• Functions
  • Arithmetics
  • Sequences
  • Iterators
  • Optionals & Errors
  • Fungible / Non-Fungible Tokens
• Generate or hand-craft index pages for each section
• Move existing function docs into their new categories
• Add cross-links and “Back to index” navigation
• Update sidebar / table of contents config
• Review for broken links or missing helpers
• (Nice-to-have) Add search tags / aliases for renamed pages