[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

Start with CONTRIBUTING.md and the possible docs/webgl_debugging.md location, then use issue #7387 and the scattergl rendering flow as context. Document WebGL trace structure, rendering entry points, regl.clear() and buffer-management patterns, plus debugging tips for rendering warnings; done means contributors can follow the workflow without extensive codebase exploration.