docs: add contributor onboarding guide for new maintainers (#320)

[annukumar123]

Description

Closes #320.

This PR introduces a comprehensive, high-quality Maintainer Onboarding Guide (docs/maintainer_onboarding.md) to help transition new maintainers smoothly onto the team.

Key additions:

• Core Mission Principles: Highlighting local-first offline privacy preservation, encouraging contributor learning, and maintaining automated check status.
• Structured Review Workflows: Defining alignment validation, sandboxed plugin execution auditing (eval/exec checks), and clean branch maintenance.
• Label Hygiene: Clarifying difficulty tags (level-1, level-2, level-3) and specific hackathon structures for SSoC 26 automatic scoring.
• Local Testing Workflows: Providing backend and frontend clean-room environment test sequences.
• Communication Guidelines: Setting expectations for responsive and educational messaging.

Hackathon Association

• Program: Social Summer of Code (SSoC) 2026
• Contributor Role: Student Participant
• Leaderboard Tracking: Please label this PR with the appropriate SSoC difficulty tags (e.g., ssoc, level-1 or Easy) so it registers for leaderboard points.

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