docs: rewrite CONTRIBUTING.md

[shiftinv]

Summary

A general overhaul of the entire contribution guide, hopefully making it easier to follow and understand. Also includes the documentation side of #196, i.e. outlining the commit/PR naming convention.

This removes the detailed example which, while insightful, was way too lengthy for new contributors. We could maybe re-add it on a separate page in the future, but for now it's too much for this guide.

preview: https://github.com/DisnakeDev/disnake/blob/docs/contributing-once-more/CONTRIBUTING.md

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)