Document listen websocket api

[jcdyer]

• Add documentation to each of the public types and methods in src/listen/websocket.rs including example code snippets and links to deepgram documentation. (See src/listen/rest.rs
• Remove the #![allow(missing_docs)] annotation from the top of that file.

Notes:

• See https://docs.rs/deepgram/latest/deepgram/listen/websocket/index.html for the current rendering of that module's docs.
• Run cargo doc --open to generate a local copy of the docs, and open them in your default browser.
• Consider pushing a patch-level release to crates.io when this is done (if no semver breaking changes have been merged yet) so the most up-to-date docs show up on docs.rs