docs: Force white background for ecosystem diagram (#564)

## Motivation and Context
The diagram looks horrible when viewed in dark mode. Re-exported with
white background enabled, preserving Excalidraw diagram source embedded.

## How Has This Been Tested?
N/A

## Breaking Changes
N/A

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

Author: iann0036