docs: align user guides with Diátaxis

[isty2e]

Summary

• turn the optimization and structured-space walkthroughs into step-by-step tutorials with expected results
• separate evaluator selection, exact-async execution, Joblib problem reuse, local-search placement, and CSA checkpointing into task-focused guides
• move execution rationale into Concepts and lifecycle, budget, failure, randomness, and persistence guarantees into Reference
• update the MkDocs navigation, guide indexes, README entry points, and affected cross-links

The top-level categories already resembled Diátaxis, but several pages mixed tutorials, operational procedures, conceptual discussion, API contracts, and benchmark interpretation. This change keeps the existing technical content while giving each page one primary reader need.

While executing the revised examples, I confirmed that variopt 0.2.0 cannot deserialize a loky request-local success carrying CandidateRefinement. The worker guide now uses the threading backend and states that limitation instead of presenting an unreliable path. Ordinary loky objective evaluation is unaffected.

Verification

• pre-commit run --all-files
• uv run --extra docs mkdocs build --strict
• uv run pytest (1710 passed)
• executed the revised Differential Evolution, structured-space, permutation, exact-async, checkpoint, and request-local examples