docs: add BiRefNet and Image Export to communityNodes.md

[veeliks]

Adds two new community nodes to the documentation:

1. BiRefNet Background Removal - High-quality background removal using BiRefNet with multiple model variants
2. Image Export - Export images in multiple formats (AVIF, JPEG, PNG, TIFF, WebP) with compression and quality controls

Nodes are published at:

• https://github.com/veeliks/invoke_birefnet
• https://github.com/veeliks/invoke_image_export

Changes include:

• Added entries to the community nodes list in alphabetical order
• Included before/after example images for BiRefNet
• Included node screenshots for Image Export
• Updated table of contents

[psychedelicious]

Thanks! In the future, please only commit non-whitespace changes (i.e. save without formatting) - it keeps the git history tidier. Not a big deal though.