docs: Rewrite AutoPopulate spec from first principles

Restructured to present DataJoint 2.0 as the status quo:

- Starts with fundamentals: table types, make() method, key_source
- Explains populate() method and operating modes
- Describes per-table jobs system as native feature
- Covers priority, scheduling, distributed computing
- Migration from 1.x moved to brief section at end

Removed problem/solution framing that assumed 1.x knowledge.
Now readable as standalone 2.0 documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko