Update SOUNDNESS.md for release v1.0.0

[rod-chapman]

For release 1.0.0, we need to update SOUNDNESS.md

Specifically:

1. Update the link to the s2n-bignum SOUNDNESS document. It moved from the "doc" subdirectory to the root of the s2n-bigum repo.
2. Update the text to cover the final set of x86_64 functions that have HOL-Light proofs at the v1.0.0 release point.

[mkannwischer]

Already implemented by #1112.