Docs: Add preprocessing conceptual guide and CLI reference

[Mikerah]

Overview

Add documentation for the new preprocessing configuration feature.

Tasks

Conceptual Guide

• Create guides/preprocessing.mdx explaining:
  • What are Beaver triples and random shares
  • Why preprocessing is needed for HoneyBadger MPC
  • How automatic analysis works
  • When to use presets vs explicit values
  • Common configurations and their use cases

CLI Reference Updates

• Update cli/overview.mdx with new flags:
  • --preprocessing
  • --beaver-triples
  • --random-shares
• Add examples of flag usage

SDK Documentation

• Update rust-sdk/api.mdx with new methods:
  • Program::preprocessing_requirements()
  • Program::count_multiplications()
  • StoffelServerBuilder::with_preprocessing_preset()
  • PreprocessingPreset enum
• Update examples section

Configuration Reference

• Document [mpc.preprocessing] section in Stoffel.toml
• Show preset and explicit value options

Files to Modify/Create

• New: guides/preprocessing.mdx
• Update: cli/overview.mdx
• Update: rust-sdk/api.mdx
• Update: docs.json (add guides section)

Reference

• RFC-0001
• Depends on: STO-377 (SDK), STO-378 (CLI)

[linear]

STO-379