Refactor kernel registration tutorials (#5297)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5297

* Added overview of existing kernel libraries.
* Separated out 2 types of APIs: yaml based API and C++ API for custom kernels.
* Made it clear which API for which use case.
* Added an CMake example for C++ API.

Reviewed By: lucylq, dbort

Differential Revision: D62546152

fbshipit-source-id: 8723f2ceef1fe6dea0acaaf634d464c3e807036f