Expand README with usage examples and project documentation #28

The README is currently a single line with no installation instructions, usage examples, or documentation links. This means the PyPI page shows "no project description" despite the README existing.

See PR #27 which fixes the metadata plumbing (readme = "README.md"), but the content itself still needs to be written.

This is related to #4 (example gallery) — once examples exist, the README should link to them. At minimum the README should include:

• What the library does and how it relates to PyTensor/PyMC
• Installation instructions
• A minimal working example (build and train a simple model)
• Links to relevant resources (docs, examples, PyTensor, PyMC)