[internal] Update pull request template for clarity and consistency

[dozro]

Description

This pull request updates the pull request template to align with the new project repository, clarify instructions, and introduce a section about generative AI usage. The changes help ensure contributors follow updated guidelines and document AI involvement in code contributions.

Template updates and clarifications:

• Updated repository and contribution guideline links to reference SableClient/Sable instead of the previous project.
• Improved the language and clarity of instructions, including the description and type of change sections.
• Revised checklist items for consistency and added a reminder to create a required changeset for changes.

Generative AI disclosure:

• Added a new section asking contributors to disclose whether generative AI was used in code contributions and to confirm adherence to project restrictions on AI-generated code.

[hazre]

Do we really need the AI disclosure part? the whole template is kinda useless as is, we don't need more stuff I feel like.

[hazre]

frankly I think we should get rid of the whole thing. or make it more actually useful. @7w1 @Just-Insane any thoughts?

[7w1]

I don’t mind getting rid of it or at least the checkbox part, none of that feels particularly helpful.

[dozro]

fine by me, just thought having a link to cinny's repo wouldn't be useful 🤷‍♀️