[FEATURE]: Improve contributor documentation for WebGL rendering and debugging workflow

Problem

While contributing a fix for issue #7387, I found that understanding the WebGL rendering flow (especially for scattergl) required significant exploration of the codebase.

There is currently limited contributor-oriented documentation explaining:

• how WebGL traces are structured
• where rendering logic is implemented
• how functions like regl.clear() are expected to behave
• how to debug rendering-related warnings

Proposed Solution

Add a short developer-focused documentation section covering:

• Overview of WebGL-based traces (e.g., scattergl)
• Key rendering pipeline entry points
• Usage patterns for regl.clear() and buffer management
• Common debugging scenarios and tips

Possible Implementation

This could be added as:

• a section in CONTRIBUTING.md
• a new markdown file (e.g., docs/webgl_debugging.md)
• or a developer note linked from the main documentation

Benefits

• Helps new contributors onboard faster
• Reduces debugging time
• Improves code maintainability
• Encourages more contributions in rendering-related areas

Context

This suggestion is based on my experience contributing to:
https://github.com/plotly/plotly.js/issues/7387

CONTRIBUTING.md と docs/webgl_debugging.md の候補となる場所から始め、issue #7387 と scattergl のレンダリングフローをコンテキストとして使用します。WebGL トレースの構造、レンダリングのエントリーポイント、regl.clear() とバッファ管理のパターン、およびレンダリング警告のデバッグのヒントをドキュメント化します。貢献者がコードベースを大幅に調査しなくてもワークフローに従えるようになれば完了です。