Update contributing.md to improve new contributor experience, add AI disclosure requirement.

[sovdeeth]

Problem

Our contributing.md file is rather outdated and misses a lot of useful information for first time contributors. This PR aims to add more information on our git setup, namely about what branches to base on (a common mistake for first time contributors) and how to build and run tests.

In addition, we have recently had a spate of new contributors who either primarily use AI to write their code, write their messages, or both. Knowing what was made by AI would help us understand issues in contributions and help guide new contributors to write more accurate code for the Skript project.

Solution

contributing.md has been edited and reworked to cut out a lot of extraneous text and provide more actionable examples for new users. I added some example git commands to provide a stepping stone for contributors unfamiliar with git, and I welcome any other suggestions for pain points.

This PR also adds a requirement to disclose AI usage when making contributions, and adds a field to the PR template for this purpose.

Any other suggestions for improving the contributing file are very welcome.

Testing Completed

N/A

Supporting Information

Thanks to biomejs, who gave me the idea for the AI disclosure requirement and from whom I pulled the base test for the disclosure section.

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Completes: none
Related: none
AI assistance: GitHub Copilot was active and helped complete some lines. All generated content was edited to ensure it was correct and fit the voice I desired.

[erenkarakal]

requested changes are up to you