Update documentation to reference p5.instance for shader context copying

[hxrshxz]

Resolves #7648

Changes:

Added documentation for P5.instance and updated copyToContext() examples to use p5.instance instead of window

Screenshots of the change:

NA

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

[Copilot]

Pull Request Overview

This PR updates the documentation for shader context copying to reference p5.instance instead of window for better clarity and correctness. The change addresses issue #7648 by providing proper documentation for the p5.instance variable and updating code examples.

• Added comprehensive documentation for the p5.instance property
• Updated copyToContext() method examples to use p5.instance instead of window

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/webgl/p5.Shader.js	Updated documentation examples to use p5.instance instead of window for copying shaders to main canvas
src/core/main.js	Added complete JSDoc documentation for the p5.instance property explaining its purpose and usage

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[perminder-17]

Hi, this has already been fixed in the dev-2.0 branch and doesn't need to have p5.Instance in the main branch instead of window, since we had change in functionality only for 2.x versions.