docs: fold reference/ into feature/, split offset-lb-frame into three EN pages, rewrite redesign.md in English

[qqeasonchen]

Follow-up to #5390 — folds the tree further per review feedback.

Motivation

Consolidate the docs layout: reference/ merges into feature/; the Chinese offset-lb-frame design page splits into three English feature pages; two architecture mechanism pages move to feature/; redesign.md becomes English.

Changes

reference/ → feature/ (7 pages)

client-java · http-api · admin-api · observability · deployment · protocols · storage-spi — all git mv'd with history.

architecture → feature (2 pages)

• connector-api-split.md — the connector API split plan (a capability/mechanism page)
• control-plane.md — delivery topology + state control plane + absorbed failure matrix

offset-lb-frame.md (中文, 391 lines) → three English feature pages

New page	Covers
feature/offset-management.md	The two offset layers (pull vs deliver/ack), per-backend mechanics table, the RocketMQ 5.x deferred-ACK chain (empopck), monotonic CAS-max writes, takeover behavior, retired designs
feature/load-balancing.md	Full-stickiness model (no forwarding), self-reported instance load (LoadMeter), the /session/recommend scoring loop, session-granularity pinning, accepted trade-offs
feature/frame-protocol.md	EventMeshFrame wire format (14B header + KV + data), msgType field mapping, FrameAdaptor SPI, WireCodec SPI, complete ingress/egress conversion chains for CloudEvents / MeshMessage / A2A, storage SPI frame handling

redesign.md → English

The 5,248-line Chinese design record is rewritten as a focused English design record: executive summary, the six iron rules (+ status snapshot), target architecture diagram, storage/SDK/ingress/egress/connector redesign summaries, the v1.2 decision log (15.1–15.8), the at-least-once vs exactly-once layer boundary, request-reply semantics, and the streaming modes — with cross-links into the current feature pages and a note that the historical appendices' live content now lives in the feature tree.

Result

docs/
├── index.md + introduction.md
├── quickstart/    getting-started · configuration
├── feature/       19 pages (core features · internals/mechanisms · API & ops reference)
└── architecture/  overview · security · redesign · review/{evidence, production-ha-plan}

Verification

• All 199 internal .md links validated resolving (0 broken) across docs/** + README.
• README documentation section updated to the new paths; index.md map rebuilt with Core features / Internals & mechanisms / API & operations groupings.