Add pull request template

[nlauchande]

Adding structure to the PR's to facilitate understanding.

Context

Gives the reviewer some context about the work and why this change is being made. Focus on the WHY from a product perspective.

Description

Provide a detailed description of how this task will be accomplished. Include technical details, steps, service integration, job logic, implementation, etc.

Changes in the codebase

Describe the functionality you are adding or modifying, as well as any refactoring or improvements to existing code.

Changes outside the codebase

Explain any changes to external services, infrastructure, third-party integrations, or database updates.

Additional information

Provide any extra information that might be useful to the reviewer, such as performance considerations or design choices.

Checklist

• Tests added/updated
• Documentation updated
• Issue referenced (e.g., Closes Model type kokoro not supported #123)