docs: reorganize data-plane and backend design by ownership

[zzylol]

Summary

Reorganize data_plane/docs and top-level docs using the same topic/ownership model as the control-plane cleanup in #444.

ASAPQuery-backend now documents only backend-owned architecture, physical runtime behavior, storage, identity, and extension boundaries. ASAPPlanner-owned parsing, query-to-summary mapping, accuracy reasoning, candidate generation, and logical selection are referenced rather than re-specified.

New structure

data_plane/docs

README.md is the index, with content split by audience:

• design_docs/query-execution.md: BackendPlan-aware ingestion, routing, readiness, summary readout, and exact fallback contracts.
• developer_docs/extension-points.md: protocol server, adapter, and fallback-client responsibilities without file-by-file recipes.
• user_guide/querying-asap.md: user-facing PromQL behavior, summary-backed results, exact fallback, accuracy, freshness, and errors.

top-level docs

• simplified getting-started overview and system architecture centered on the OTel summary pipeline;
• design_docs/summary-storage.md: active materialization storage and lifecycle contract;
• design_docs/series-identity.md: active canonical sid contract and recovery behavior;
• design_docs/future-storage-and-compression.md: dormant/non-MVP persistence, compaction, backfill, service mode, compression, sampling, and profiling scopes;
• developer_docs/adding-summary-family.md: cross-repository ownership and backend validation checklist.

Removed

• historical PromSketch and SID-unification migration plans;
• three implementation-specific extension walkthroughs;
• obsolete query-log bootstrap instructions that place planning in ASAPCollector;
• duplicated controller/planner, arithmetic, precompute, Sketch DB, persistence, pluggability, roadmap, compression, benchmark-estimate, and proof documents;
• stale root TODO.md, which duplicated deleted progress/roadmap material and referenced removed files.

Relevant source comments now point to the consolidated storage and identity documents.

Validation

• git diff --check
• all local Markdown links resolve across docs, data_plane/docs, control_plane/docs, and the root README;
• retained design documents declare status and MVP relationship;
• searched the repository for names of all removed documents;
• cargo fmt --all -- --check was attempted but is blocked by pre-existing formatting differences in sibling ASAPCollector/asap-precompute-rs; no Rust behavior changed in this PR.