chore: restructure README around quickstart and guard policies

[lpcox]

Reorganize README from 333 to 165 lines with focused structure:

• Quick Start (Docker) stays prominent at top
• Guard Policies section with allow-only and write-sink examples
• Architecture diagram with transport/routing/security/logging summary
• API Endpoints (brief)
• Further Reading table linking to all detailed docs

Move TOML config format, JSON examples, usage CLI flags, env vars, containerized mode, logging details, auth, and compatibility info into docs/ sub-documents or the Further Reading links.

Add TOML format section and usage examples to docs/CONFIGURATION.md.

[Copilot]

Pull request overview

Restructures the top-level README into a shorter, quickstart-first document and moves detailed configuration/usage material into the docs, adding a new “Configuration Formats” section and examples to the configuration reference.

Changes:

• Reorganized README.md around Quick Start, Guard Policies, Architecture, API Endpoints, and Further Reading links.
• Expanded docs/CONFIGURATION.md with JSON-vs-TOML format guidance, examples, validation notes, and CLI usage.
• Consolidated references to detailed topics (env vars, sessions, security model, etc.) via a “Further Reading” table in the README.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/CONFIGURATION.md	Adds configuration-format overview, TOML/JSON examples, validation notes, and CLI usage guidance.
README.md	Replaces long-form documentation with a structured quickstart + guard policies + links to detailed docs.

Comments suppressed due to low confidence (1)

docs/CONFIGURATION.md:69

• This JSON example also omits how the guard itself is enabled. guard-policies alone doesn’t select a non-noop guard for allow-only; users need to configure a guard (e.g., "guard": "github-guard" plus a guards entry) or rely on discovered per-server WASM guards. Consider adding a minimal guard/guards snippet or a short note here to prevent configs that appear valid but don’t actually enforce DIFC.

  "mcpServers": {
    "github": {
      "type": "stdio",
      "container": "ghcr.io/github/github-mcp-server:latest",
      "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": ""
      },
      "guard-policies": {
        "allow-only": {
          "repos": ["github/gh-aw-mcpg", "github/gh-aw"],
          "min-integrity": "unapproved"
        }
      }
    },

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