docs: Add standardized Pull Request template

[ayushsri-dev]

Description

Introduces a standardized GitHub Pull Request template (.github/PULL_REQUEST_TEMPLATE.md) to guide contributors when submitting changes to Arvio.

Related Issue

Closes #482

Type of Change

• Bug fix
• New feature
• UI/UX improvement
• Refactor / performance improvement
• Documentation
• Build / CI / tooling

Key Changes

• Created .github/PULL_REQUEST_TEMPLATE.md to establish a clear structure for incoming PRs.
• Added prompts for issue linking (Closes #), change categorization, and implementation highlights.
• Included target platform verification options (Android TV, Mobile/Tablet, iOS) and Gradle build checks (./gradlew assembleSideloadDebug).
• Added explicit checklist verification for compliance with Arvio's README.md Content Policy.

Testing

• Android TV / Fire TV (D-pad / remote navigation)
• Mobile / Tablet (touch UI)
• iOS
• ./gradlew assembleSideloadDebug completed successfully
• Manual testing performed on a target device or emulator
• Not applicable

Verified markdown rendering and structure of the template file locally.

Screenshots / Demo

N/A (Non-UI documentation update)

Checklist

• My changes follow the repository's existing code style and conventions.
• I have reviewed my own changes.
• My changes do not include credentials, private keys, or prohibited third-party media links as defined by the README.md Content Policy.
• I have updated relevant documentation where needed.
• I have kept this PR focused and avoided unrelated changes.