Migrate removed docs to README

[Savio-Sou]

Problem

When this library was spun out from the Noir standard library as of noir-lang/noir#6591, the existing documentations of this library was removed from the Noir docs without relocation to a readable place.

Happy Case

Copy information https://github.com/noir-lang/noir/blob/df8f2eee5c27d3cd4b6128056afdd9bd4a0322fe/docs/docs/noir/standard_library/cryptographic_primitives/eddsa.mdx that is relevant still into the README of this repository.

Project Impact

Nice-to-have

Would you like to submit a PR for this Issue?

No

[Savio-Sou]

@TomAFrench please reassign to an appropriate teammate if applicable, thanks!