Documentation spread across multiple repos

Documentation across the project is currently scattered across multiple tools and locations. This creates friction for the everyone trying to find canonical information.

### Proposal
The goal is to consolidate everything into a single, unified human-authored layer. This issue consolidates all human-authored documentation into a single site using Mintlify (prose committed as markdown).

**In scope:** Consolidating human-authored documentation from ~10 repos into Mintlify.
**Out of scope:** Python docstrings (Sphinx) and Rust API docs (docs.rs) — these automated layers remain separate and are not being migrated.

### Plan
- Audit existing documentation locations across all repos
- Define structure and taxonomy for the Mintlify site
- Migrate human-authored content into Mintlify
- Link to Sphinx and docs.rs from Mintlify where relevant
- Communicate the new location to contributors