Align documentation with TypeScript SDK structure

[nficano]

Goal

Align this SDK's docs/ directory with the canonical layout used by the TypeScript SDK. The conceptual pages (getting-started, architecture, transports, guides, etc.) are identical across every SDK. The per-source-unit subdirectory adapts to whatever the language calls its organizational unit — and only exists if this SDK actually has multiple such units.

The TS SDK uses docs/packages/ because it is a TypeScript monorepo with multiple npm packages. Do not blindly copy that name. If this SDK has multiple .NET projects, use docs/projects/. Multiple Gradle modules → docs/modules/. Multiple SwiftPM targets → docs/targets/. One single artifact → omit the subdirectory entirely — there is nothing to mirror.

The TS docs/README.md shape (Start here / Guides / Packages / Reference / Diagrams) is the index template — replace its "Packages" section with whatever this SDK's source-unit name is, or remove it if this SDK has only one artifact.

Current state (php-sdk)

• Already very close to target layout — has README.md, getting-started.md, architecture.md, transports.md, cli.md, recipes.md, conformance.md, troubleshooting.md, and all 10 guides/ pages.
• This SDK ships a single composer package (arcp/arcp). There is no multi-package equivalent to mirror, so no per-source-unit subdirectory is needed.
• Existing related tickets: Build out 10 conceptual guides matching TS SDK #23 (10 conceptual guides — likely partially done).

Target structure

docs/
├── README.md                       # index, links to everything below
├── getting-started.md              # install + minimal client+runtime example
├── architecture.md                 # how the modules fit together (references docs/diagrams/architecture-*.svg)
├── transports.md                   # WebSocket, stdio, in-memory; selection guide
├── cli.md                          # the `arcp` binary (omit only if SDK ships no CLI)
├── recipes.md                      # copy-paste solutions to common problems
├── conformance.md                  # spec coverage by section
├── troubleshooting.md              # error codes + fixes
├── guides/
│   ├── sessions.md                 # spec §6
│   ├── resume.md                   # spec §6.3
│   ├── auth.md                     # spec §6.1
│   ├── jobs.md                     # spec §7
│   ├── job-events.md               # spec §8
│   ├── leases.md                   # spec §9
│   ├── delegation.md               # spec §10
│   ├── observability.md            # spec §11
│   ├── errors.md                   # spec §12
│   └── vendor-extensions.md        # spec §15
└── diagrams/                       # graphviz sources + rendered light/dark SVGs

Per-source-unit pages

N/A — no subdirectory. This SDK has one composer package, so there is nothing to mirror. Document the public API surface inside docs/architecture.md (top-level namespace tour) and in the relevant guides; do not invent a docs/packages/ or docs/namespaces/ directory just to fill a slot.

If a single public namespace ever splits out into its own composer package, add a subdirectory then.

Authoring rules

• Guides are spec-aligned, not feature-aligned. If the SDK currently has features/heartbeat.md, features/ack.md, etc., fold them into guides/job-events.md. If it has features/cost-budget.md, fold into guides/jobs.md (or its own guides/budgets.md only if it would otherwise be >2 screens).
• docs/README.md is the index. Four sections (or three if no multi-artifact split): Start here, Guides, (if applicable), Reference. No content lives only in the index.
• Diagrams use <picture> with prefers-color-scheme and ship both light + dark SVGs. Reference the architecture diagram from docs/README.md and docs/architecture.md.
• Spec links point at ../../spec/docs/draft-arcp-1.1.md#section so they survive renames.
• No orphan pages. Anything not linked from docs/README.md must be deleted or linked.
• CONFORMANCE.md at repo root stays authoritative; docs/conformance.md summarizes + links to it.

Scope

This SDK is the closest to the canonical shape — the remaining work is small:

1. Audit each existing top-level page against TS counterpart and bring shape into alignment.
2. Verify docs/README.md index matches the TS shape, minus the Packages section (this SDK has one artifact).
3. Consider deleting root-level PHP_SDK_GUIDE.md if its content now lives in docs/.

Acceptance

• Every conceptual file in the target structure exists (except cli.md if this SDK truly has no CLI — note the omission in docs/README.md)
• docs/README.md matches the four-section shape of TS docs/README.md, substituting this SDK's source-unit term (or omitting that section entirely if single-artifact)
• Every page is linked from docs/README.md; no orphans
• Old feature-shaped pages (features/*, concepts/*, numeric NN-*.md) deleted or merged into canonical pages
• Diagrams use <picture> light/dark and render on GitHub

Closes / supersedes

• Subsumes Build out 10 conceptual guides matching TS SDK #23 (10 conceptual guides)

Reference

• TS SDK docs tree: https://github.com/agentruntimecontrolprotocol/typescript-sdk/tree/main/docs
• TS docs/README.md (index shape to copy, minus the language-specific "Packages" wording): https://github.com/agentruntimecontrolprotocol/typescript-sdk/blob/main/docs/README.md
• Spec: ../spec/docs/draft-arcp-1.1.md