Add Custom Terrain Materials documentation page

[julia-rbx]

Summary

• Adds a new content/en-us/parts/terrain-materials.md page covering the 62-slot Expanded Terrain system
• Content focuses exclusively on topics not already documented: enabling ExpandedTerrain, programmatic Terrain API (SetMaterialSlot, GetMaterialSlot, ResetMaterialSlot, WriteVoxelChannels), performance improvements, known issues, and terrain roadmap
• Cross-links to parts/materials.md for Material Manager and MaterialVariant workflows instead of duplicating that content
• Adds navigation entry in content/common/navigation/engine/guides.yaml under the existing Terrain entry
• Adds a cross-link from the "Terrain materials" section of parts/terrain.md to the new page

Overlap analysis

Verified by grep that none of the new API methods (SetMaterialSlot, GetMaterialSlot, ResetMaterialSlot, WriteVoxelChannels) or the ExpandedTerrain property appear on any existing page. Sections covering Material Manager, MaterialVariant creation, and Material Generator are intentionally omitted (cross-linked only) since they are already well-documented in parts/materials.md.

Test plan

• Verify the new page renders correctly in the docs site
• Confirm navigation entry appears under Parts > Terrain
• Confirm cross-links from terrain.md and within the new page resolve correctly
• Review code sample for accuracy against the current Terrain API