ETRecord OSS documentation (#510)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/510

This should be put under the sdk part, in parallel with the etdump documentation under a section maybe named "Artifacts for Inspection" (we can decide on the naming later)

Most content comes directly from https://www.internalfb.com/intern/staticdocs/executorch/docs/sdk/generating_etrecord/ and the ETrecord part of https://www.internalfb.com/intern/staticdocs/executorch/docs/sdk/getting_started/#etrecord

Reviewed By: tarun292

Differential Revision: D49712632

fbshipit-source-id: 263edd4d5dea6628e7f0025e7a3c3c9c3eb4a306