[AAP-64338] Enhance documentation for Gateway API plugin

[nickbhasin]

Updated documentation to clarify plugin usage and warnings.

Description

• What is being changed?
  Updated ansible.platform.gateway_api doc. It is designed to support Gateway API endpoints
• Why is this change needed?
  Customers were using it to querying APIs outside of the Gateway API
• How does this change address the issue?
  Added necessary info the documentation

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test update
• Refactoring (no functional changes)
• Development environment change
• Configuration change

Self-Review Checklist

• I have performed a self-review of my code
• I have added relevant comments to complex code sections
• I have updated documentation where needed
• I have considered the security impact of these changes
• I have considered performance implications
• I have thought about error handling and edge cases
• I have tested the changes in my local environment

Testing Instructions

Prerequisites

Steps to Test

Expected Results

Additional Context

Required Actions

• Requires documentation updates
• Requires downstream repository changes
• Requires infrastructure/deployment changes
• Requires coordination with other teams
• Blocked by PR/MR: #XXX

Screenshots/Logs