docs(optimizers): add DSPy optimization docs page and example notebook

PR 3 of the DSPy prompt optimization feature: user-facing documentation
and a runnable end-to-end example demonstrating the optimizer API.

Changes:
- docs/en/concepts/dspy-optimization.mdx: Mintlify page covering quickstart,
  sequence diagram, algorithm comparison, metric writing guide, API reference,
  observability, limitations, and troubleshooting
- examples/dspy_optimization.ipynb: 9-cell notebook with a two-agent
  email-drafting crew, pairwise LLM-judge metric, and 10 training examples
- docs/docs.json: add dspy-optimization to Core Concepts navigation (between
  training and memory)
- lib/crewai/src/crewai/optimizers/dspy_optimizer.py: drop redundant
  type: ignore[import-not-found] comment that caused mypy unused-ignore error

All 28 optimizer unit tests pass. ruff clean. mypy clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mohamedalishiha