docs : add AGENTS.md and CONTRIBUTING.md [no ci] (#3826)

docs : add AGENTS.md and CONTRIBUTING.md [no ci] (#3826)

* docs : add AGENTS.md and CONTRIBUTING.md [no ci]

This commit add AGENTS.md and CONTRIBUTING.md which are based on the
same files in llama.cpp. They have been modified slightly to fit with
whisper.cpp.

The motivation for this is to clarify the contribution policy in
whisper.cpp so that contributers can have a better understanding of the
expectations and requirements for contributing to the project.