Document gradient fills

[brake-boy]

Change Summary

Adds gradient fill documentation to the Shapes API and Type Interfaces pages.

Change Description

• Documents linear and radial gradient fills
• Adds examples for shapes, lines, and slide backgrounds
• Documents per-stop transparency
• Documents angle, scaled, and rotateWithShape
• Adds reference tables for ShapeGradientProps and ShapeGradientStopProps
• Clarifies that ShapeLineProps inherits gradient options from ShapeFillProps
• Documents the solid-fill fallback when fewer than two stops are provided

Change Type

• Bug fix
• New feature
• Documentation update

Related Issue

Companion documentation for #1469, which adds gradient fill support and addresses #102 and #93.

Motivation and Context

The gradient API introduced by #1469 needs user-facing documentation outside the main README and the demo slides. This update places usage examples on the Shapes API page and documents the complete public interface on the Type Interfaces page.

Checklist before requesting a review

• If it is a core feature, I have added new code under /demos/modules/ (included in companion PR Add gradient fill support #1469)
• My code follows the style guidelines of this project
• My changes generate no new eslint warnings (documentation-only change)
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas (not applicable to this documentation-only change)
• I have included code/tests that prove my fix is effective or that my feature works (production Docusaurus build passes)
• I have used the "Run All Demos" feature on the browser demo and no errors were found (verified for companion PR Add gradient fill support #1469)

Screenshots / Sample Code

The Shapes documentation includes examples for linear gradients, radial gradients, per-stop transparency, gradient lines, and slide backgrounds.

Thanks for your contribution!