Post release doc updates

[samaloney]

PR Description

Update readme

• Fix Zenodo badge use latest DOI ref (alway point to most recent release)
• Add short pip install instructions

Update install guide

• Add pip install section and why legacy might be needed.

AI Assistance Disclosure

AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

Regardless of AI use, the human contributor remains fully responsible for correctness, design choices, licensing compatibility, and long-term maintainability.