docs: reorganize ASAP-aware mapping documentation

[milindsrivastava1997]

Summary

• split the monolithic ASAP-aware mapping design document into focused topic documents
• add a top-level mapping README with links and descriptions for every detailed design file
• update repository, developer-guide, and user-guide links to the new documentation index

Validation

• verified all detailed-design link targets exist
• verified no Markdown files reference the removed asap_aware_mapping.md path
• passed repository pre-commit hooks

[zzylol]

LGTM

Some TODOs:

• add more optimization/replacement strategies, and the algorithms for searching optimal cost with these strategies, the complexity of the search.
• add cost definitions, optionally with a doc for cost per summary family
• add accuracy/error propagation design doc when handling nested summaries

[zzylol]

Follow-up tracking issue: #301 covers the remaining TODOs for additional optimization/replacement strategies and optimal-cost search complexity, canonical cost definitions, and nested-summary accuracy/error propagation design.