groth16: add high-level wrapper API + end-to-end tutorial

[0xpantera]

Summary

• add high-level Groth16 wrapper API in GrothProofs:
  • setup, prove, process_vk, verify, verify_prepared
  • conventions helpers: validate_witness_shape, public_inputs_from_witness
• keep existing low-level API (setup_full, prove_full, verify_full) intact
• add wrapper/convention test coverage
• add end-to-end docs page with explicit public-input/witness conventions

Why

This makes the package easier to adopt for Groth16 workflows by providing a single ergonomic flow that mirrors arkworks-style usage while preserving existing internals.

Validation

• julia --project=GrothProofs -e 'using Pkg; Pkg.test()'
• julia --project=. docs/make.jl

Commits

1. groth16: add high-level wrapper API and conventions
2. groth16: add wrapper and convention tests
3. docs: add Groth16 end-to-end wrapper tutorial