Fix links and references.
Fix links and references.

- Use https://web-payments.org/ base.
- Update to link to secure-messaging spec.
- Use AES vs FIPS-197.
- Fix typos.

BrendanBenshoof/website,BrendanBenshoof/website,bsletten/website,BrendanBenshoof/website,opencreds/website,opencreds/website,bsletten/website,bsletten/website,opencreds/website