docs(optimizr): add logo and fix rtd deps

docs/source/algorithms/sparse_optimization.md

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+# Sparse Optimization
+
+Sparse PCA and Elastic Net utilities with Rust speed.
+
+## Sparse PCA
+
+```python
+import numpy as np
+from optimizr import sparse_pca_py
+
+X = np.random.randn(500, 20)
+components = sparse_pca_py(X, n_components=5, l1_ratio=0.15)
+print(components.shape)  # (5, 20)
+```
+
+## Elastic Net
+
+```python
+import numpy as np
+from optimizr import elastic_net_py
+
+X = np.random.randn(200, 8)
+y = np.random.randn(200)
+coeffs = elastic_net_py(X, y, l1_ratio=0.3, alpha=0.01)
+print(coeffs)
+```
+
+## Notes
+- Inputs should be NumPy arrays; data is copied to Rust.
+- `l1_ratio` balances sparsity vs ridge penalty.
+- Standardize features before calling for stable solutions.