Improve docs SEO metadata (#8)

Author: sjquant

docs/api/background.md

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+---
+description: Reference for QuickThumb background layers, including colors, gradients, images, opacity, effects, and render order.
+---
+
 # Background
 
 `.background()` adds a full-canvas background layer. Multiple background calls stack in render order — first call is backmost.

docs/api/canvas.md

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+---
+description: Reference for QuickThumb Canvas creation, aspect ratios, layer composition, rendering, JSON loading, and image output.
+---
+
 # Canvas
 
 The `Canvas` is the root object. It holds dimensions and an ordered list of layers.

docs/api/effects.md

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+---
+description: Reference for QuickThumb effects, including filters, shadows, strokes, blend modes, opacity, and layer compatibility.
+---
+
 # Effects
 
 Effects are modifiers applied to layers. Each layer type accepts a specific set.

docs/api/enums.md

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+---
+description: Reference for QuickThumb enums and gradient models used for alignment, fit modes, shapes, filters, and color transitions.
+---
+
 # Enums & Gradients
 
 ## Align

docs/api/image.md

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+---
+description: Reference for QuickThumb image layers, including local and remote images, fit modes, positioning, masking, effects, and background removal.
+---
+
 # Image
 
 `.image()` adds a positioned overlay image. Supports local files, remote URLs, background removal, fit modes, blend modes, and effects.

docs/api/index.md

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+---
+description: Explore the QuickThumb API reference for Canvas, layers, backgrounds, text, images, shapes, outlines, effects, enums, and gradients.
+---
+
 # API Reference
 
 Complete reference for every class, method, and parameter in QuickThumb.

docs/api/outline.md

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+---
+description: Reference for QuickThumb outline layers that draw canvas borders with configurable width, color, opacity, and corner radius.
+---
+
 # Outline
 
 `.outline()` draws a border around the entire canvas edge. It is typically added as the last layer so it sits on top of all content.

docs/api/shape.md

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+---
+description: Reference for QuickThumb shape layers, including rectangles, ellipses, fills, gradients, strokes, shadows, and positioning.
+---
+
 # Shape
 
 `.shape()` adds a positioned rectangle or ellipse.

docs/api/text.md

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+---
+description: Reference for QuickThumb text layers, including rich text parts, fonts, alignment, wrapping, strokes, shadows, and sizing.
+---
+
 # Text
 
 `.text()` adds a text layer. Content can be a plain string or a list of `TextPart` objects for per-segment styling.

docs/changelog.md

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+---
+description: Review QuickThumb release notes and follow version changes, fixes, and new features through GitHub Releases.
+---
+
 # Changelog
 
 Release notes are published on GitHub Releases: