Document the BioMixer replacement visualizer

[alexandrutodor]

Summary

• document the maintained BioMixer replacement visualizer
• describe bundled, standalone, and Helm deployment options
• cover feature enablement, image-access prerequisites, verification, and rollback

The replacement implementation is tracked in ontoportal/ontoportal_web_ui#74 and the standalone source is https://github.com/ontoportal/biomixer-visualizer. Deployment integration is being submitted separately.

Validation

• front matter and links parsed successfully
• modern-Ruby bundle exec jekyll build
• bundle exec rake search:init
• git diff --check