[docs] Refine community contribution process

[AsakuraMizu]

We are currently missing refined guidelines for community contributions. To better welcome community contributors and reduce the mental burden on maintainers, we need to standardize the contribution process.

This includes:

Setup Issue & PR templates

GitHub documentation: Using templates to encourage useful issues and pull requests

Add CONTRIBUTING.md

GitHub documentation: Setting guidelines for repository contributors

Key notes:

• Pass clippy test (also checked in CI) and format your code (currently no check in CI)
• Follow Conventional Commits specification
• Split PRs as small as possible for easier review
• Maintain a linear history for better readability. Avoid rebasing too many times to keep the history clean, as we'll eventually squash. The only exception is to use rebase instead of merge when synchronizing the progress of the main branch.

Take https://github.com/compio-rs/compio/blob/master/CONTRIBUTING.md or https://github.com/tokio-rs/tokio/blob/master/CONTRIBUTING.md as a good reference.