Enhance font hinting and ANI documentation in ImageSharp

[JimBobSquarePants]

This pull request updates and expands the documentation for SixLabors.Fonts, introducing new features and clarifying existing behaviors related to color fonts, fallback font resolution, font metrics, hinting, and custom rendering. The most significant changes include new documentation for font palettes and fallback resolvers, a comprehensive rewrite of the font hinting section to explain multiple hinting modes, and updates to custom rendering APIs and examples.

Color Font and Palette Enhancements:

• Added documentation for COLR font palettes, including how to select and override palettes with TextOptions.FontPalette, FontPaletteOverride, and related APIs.
• Clarified how COLR v1 composite groups are rendered using BeginGroup/EndGroup in custom renderers, and updated references from ClipQuad to ClipBounds. [1] [2]

Font Fallback Improvements:

• Introduced resolver-based fallback with TextOptions.FontFallbackResolver, including usage of SystemFonts.FallbackResolver and custom resolvers via IFontFallbackResolver. Updated shaping documentation to reflect the new fallback resolution order. [1] [2] [3]

Font Hinting Documentation Rewrite:

• Rewrote the hinting documentation to describe both TrueType and CFF hinting, and introduced HintingMode.Standard (vertical-only) and HintingMode.Full (classic bi-level) with detailed explanations of their effects and tradeoffs. [1] [2] [3] [4]
• Updated the documentation index and section headings to reflect the broader scope of "Font Hinting" beyond just TrueType.

Custom Rendering API and Example Updates:

• Updated custom rendering documentation to use TextRenderer.RenderTo instead of RenderTextTo, added BeginGroup/EndGroup to the IGlyphRenderer example, and clarified that GlyphRendererParameters now includes HintingMode for cache key purposes. [1] [2] [3] [4]

Font Metrics Clarification:

• Added documentation for FontMetrics.XHeight and FontMetrics.CapHeight to clarify how lowercase and capital letter heights are represented.