Adds pull request template (#3943)

## Summary
Introduces a new pull request template for the Elastic Docs repository
that includes required GenAI disclosure questions. The template is
designed to help internal and external contributors provide clear and
accurate documentation updates while ensuring transparency around
AI-assisted contributions.

## What’s changing

- Adds a new PR template to .github/PULL_REQUEST_TEMPLATE.md.
- Incorporates two GenAI disclosure questions drafted by Legal:
     - Whether a GenAI tool was used.
     - Which tool(s) and model(s) were used, if applicable.

## Why this change is needed

The Elastic Legal and Open Source teams have requested that all
public-facing repositories include standardized questions about
generative AI usage in documentation contributions. These questions help
us:

- Maintain transparency regarding AI-assisted content.
- Support responsible and high-quality use of AI for documentation.
- Ensure reviewers have the context needed to evaluate contributions.
- Establish consistency across Elastic open repositories, particularly
as external contributors begin using AI more frequently.

Adding this template to the Elastic Docs repo also helps contributors
understand our expectations upfront and aligns with the AI & Docs
Guidelines we’ve developed internally.

## What's needed from reviewers

- Content accuracy and clarity: Does the wording in the template feel
clear, welcoming, and consistent with our documentation standards?
- Completeness: Are there additional checklist items or guidance we
should add for contributors?

## Additional question

Should we also include a separate template specifically for external
contributors, or is the single unified template sufficient?

Author: KOTungseth

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,24 @@
+<!--
+Thank you for contributing to the Elastic Docs! 🎉
+Use this template to help us efficiently review your contribution.
+-->
+
+## Summary
+<!--
+Describe what your PR changes or improves.  
+If your PR fixes an issue, link it here. If your PR does not fix an issue, describe the reason you are making the change. 
+-->
+
+## Generative AI disclosure
+<!--
+To help us ensure compliance with the Elastic open source and documentation guidelines, please answer the following:
+-->
+1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?
+- [ ] Yes  
+- [ ] No  
+<!--
+2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).
+
+Tool(s) and model(s) used:
+-->
+