[webauthn] enrichen registration and authentication ceremonies definitions

equalsJeffH has just created a new issue for https://github.com/w3c/webauthn:

== enrichen registration and authentication ceremonies definitions ==
In various places we are appropriately referring readers to the registration and authentication ceremonies definitions. 

However, those definitions are very high-level and themselves do not link back into the spec such that a reader (e.g. a web developer) might be able to figure out how implementing a ceremony relates to the actual JS they need to write and the API calls they need to make.

e.g., add links to:
* [[WebAuthnAPIGuide]] (ie: https://developer.mozilla.org/en-US/docs/Web/API/Web_Authentication_API)
* https://w3c.github.io/webauthn/#use-cases
* https://w3c.github.io/webauthn/#sample-scenarios
* https://w3c.github.io/webauthn/#api  (introduction)

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/1099 using your GitHub account