docs: Rewrite spec with context-based approach

New approach using dj.new() for isolated contexts:
- Each context has one config and one connection
- ctx.Schema() auto-uses context's connection
- ctx.Manual, ctx.Lookup, etc. for table base classes
- dj module acts as singleton context (legacy API)
- thread_safe=True blocks singleton, allows dj.new()

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

Author: dimitri-yatsenko