Remove etrecord parse_etrecord API from doc (#805)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/805

Chatted about this with Tarun. We think the ETRecord object should be opaque to the user so they shouldn't need this API. The format is also matching with the etdump page better.

Reviewed By: tarun292

Differential Revision: D50140990

fbshipit-source-id: 14d01ec85bda70ff022f52b2196bd1635ecba54b