Enhance Go+ contributing guidelines and introduce code of conduct

[aofei]

Proposal

Introduce a code of conduct and revise the contributing guidelines to streamline the contribution process, enhance inclusivity, and improve commit message clarity across the Go+ project.

Background

The Go+ project has grown significantly, with contributions spanning multiple repositories. However, the absence of a unified code of conduct and inconsistent contributing guidelines across repositories have led to challenges. These include a lack of a standardized approach to contributions and difficulties in understanding the project's current developments due to unclear commit history.

Workarounds

N/A

[aofei]

Due to some tasks still pending completion (primarily updates to the goplus.org website), this issue cannot yet be closed.

I'm currently occupied with writing the language spec and will revisit this once that work is completed.