docs: add Extending Libraries section with NumSharp.Bitmap

Reorganize documentation structure:

Removed:
- NDArray.Creation.md (sparse content, API reference sufficient)
- NDArray.LinAlg.md (sparse content, API reference sufficient)

Added:
- extensions/index.md - Overview of extension packages
- extensions/bitmap.md - Comprehensive NumSharp.Bitmap documentation
  - Installation and platform notes
  - Bitmap.ToNDArray() with all parameters explained
  - NDArray.ToBitmap() with format handling
  - BitmapData.AsNDArray() for zero-copy access
  - Common patterns (ML preprocessing, grayscale, batch)
  - Memory layout (BGR/BGRA order)
  - Known limitations and workarounds
  - API reference tables

TOC structure:
- Introduction
- NumPy Compliance & Compatibility
- Array API Standard
- Extending Libraries (collapsed)
  └── NumSharp.Bitmap

Updated homepage to link to compliance page instead of removed
Array Creation page.

Author: Nucs