Add Documenter site, polished docstrings, doctests, and benchmark write‑ups

[0xpantera]

Summary

• cleaned and standardised docstrings across GrothAlgebra, GrothCurves, and GrothProofs, removing the legacy FieldElement.jl.
• introduced a full Documenter setup (docs project, make.jl, Manual pages for algebra/curves/proofs/implementation notes, RareSkills + arkworks mapping, and benchmark content with images + JSON examples).
• added seeded doctested examples for core APIs and the Groth16 pipeline to ensure docs stay executable.
• configured the GitHub Actions “Documentation” workflow to build and deploy to gh-pages, include plots/assets, and support future hosting automation.

Testing

• julia --project=docs docs/make.jl