docs: restructure README around progressive disclosure

Reorganize for first-time / beginner users:

- Title, badges, architecture image and a one-line tagline stay at the top.

- Collapse Terms of use under an IMPORTANT callout so it's prominent but out of the way.

- Collapse Prerequisites.

- Lead with three Quickstarts in a single section:

    1. Zero-touch in GitHub Codespaces (no install)

    2. Ask an AI agent (single one-shot prompt + optional add-ons, with Agent setup collapsed)

    3. Locally with azd up

- Add a short Use Claude section (CLI + SDK + Verify one-liner) and a short Tear down section.

- Move everything else under an Advanced section as collapsed <details>: model choice, full env-var table, Claude Code post-deploy details, workspace-scoped az login, manual verify checks, SDK shape + token refresh shim, API key alternative, what gets deployed, repo layout, RBAC details + grant one-liners, preflight, quota script, soft-deleted account purge, modelProviderData.

- Troubleshooting table itself also collapsed under its anchor.

- All existing anchors preserved (#terms-of-use, #configuration, #required-permissions, #troubleshooting, #granting-data-plane-roles-after-azd-up, #free-quota-held-by-soft-deleted-accounts, #preprovision-preflight-marketplace-catalog--quota, #advanced-check-claude-quota--capacity-programmatically, #advanced-long-running-processes-auto-refreshing-the-entra-id-token, #workspace-scoped-az-login, #verify-claude-code-is-wired-up, #quickstart-in-codespaces, #choosing-which-models-to-deploy, #claude-code-post-deploy-setup) so SKILL.md and external links continue to resolve.

Author: achandmsft