v0.4.0: Experimental PlonK support and user documentation

[gbotrel]

v0.4.0 milestone.

In particular, this PR adds:

• Experimental support for PlonK zk-SNARK (Experimental PLONK support #67): it this release, PlonK is not optimized and commitment-agnostic.
• User documentation (Readthedocs live user documentation website #68)

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Release notes:

v0.4.0

Docs

• added gnark User Documentation
• updated Go package documentation

Feat

• gnarkd: exposing gnark APIs through RPCs (#54)
• PlonK: adding functionality to convert a constraint system to PlonK constraints (#56)
• PlonK: added experimental support for PlonK backend

Fix

• inverse and div in frontend had some variable ID offset issues (#62)
• cs.Println doesn't trigger panic anymore
• Split S in EdDSA signature to prevent overflow #88
• fft: fixed the ordering of cosets factor according to DIF/DIT

Groth16

• VerifyingKey data structure change to ensure compatibility with other impl and Solidity in Ethereum. Serialization format change.

Integration_test

• added witness serialization tests

Refactor

• gurvy -> gnark-crypto
• moved fft in gnark-crypto
• bn256 -> bn254, bls377 -> bls12-377, etc. following gnark-crypto v0.4.0
• removed the Curve field in the R part of eddsa signature

Test

• added frontend and backend fuzz.go, go-fuzz compatible format
• added cs.Println must not panic base test

Build

• moved solidity integration tests in github.com/consensys/gnark-tests
• added integration fuzz test in backend/groth16/fuzz_test.go