Docs: Revamp getting started, prerequisites, and site IA for onboarding

### Doc area

User guide (`docs/guides/USER_GUIDE.md` or synced site)

### Describe the issue

Customers evaluating ABCA are **not completing Quick Start successfully**. Feedback (including [#17](https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/issues/17)) shows they bounce between four+ doc pages, miss mandatory fork/CDK wiring steps, underestimate AWS prerequisites, and abandon before submitting a first task. The static docs site ([Starlight](docs/astro.config.mjs)) needs clearer getting-started narrative, stronger visual warnings, and new content categories (use cases, tutorials, customization).

### Affected docs

**Source (edit these; run `mise //docs:sync`):**

- [docs/guides/QUICK_START.md](docs/guides/QUICK_START.md) → synced to `getting-started/quick-start`
- [docs/guides/DEVELOPER_GUIDE.md](docs/guides/DEVELOPER_GUIDE.md) — repository preparation section
- [docs/guides/USER_GUIDE.md](docs/guides/USER_GUIDE.md) — authentication / onboarding cross-links
- [docs/astro.config.mjs](docs/astro.config.mjs) — sidebar structure (new sections)

**Published site:** https://aws-samples.github.io/sample-autonomous-cloud-coding-agents/getting-started/quick-start/

### Suggested change

#### 1. Quick Start — "What happens" overview (top of page)

Add a short paragraph + simple diagram **before** Step 1 explaining the end-to-end journey:

```mermaid
flowchart LR
A[Install toolchain] --> B[Fork sample repo + PAT]
B --> C[CDK deploy platform]
C --> D[Store token + Cognito user]
D --> E[Configure CLI + submit task]
E --> F[Agent opens PR]
```

Call out ~30 min total, one AWS account, one forked repo. Link to the post-deploy setup improvement ([#17](https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/issues/17)) as future `mise run setup` / `bgagent setup`.

#### 2. Prerequisites — expand AWS & GitHub requirements

Add explicit callouts:

| Requirement | Why |
|-------------|-----|
| **GitHub account** with ability to fork `awslabs/agent-plugins` and create a fine-grained PAT | Agent needs write access to *your* fork |
| **VPC-capable deploy role** | Stack creates a VPC (private subnets, NAT, endpoints, DNS Firewall). Deploy principal needs EC2/VPC create permissions per [DEPLOYMENT_ROLES.md](docs/design/DEPLOYMENT_ROLES.md) |
| **Bedrock model access** | IAM + console subscription (already partially documented post-Step 3 — move summary up) |
| **Docker** | AgentCore image build during deploy |
| **`ec2:DescribeAvailabilityZones`** | Required at `mise run build` / synth time (already a Note — promote to prerequisites table) |

Link to generated least-privilege bootstrap policies (`mise //cdk:bootstrap`) instead of implying AdministratorAccess.

#### 3. Prepare repository — mandatory fork warning

In **Step 2**, add a prominent admonition (Starlight `:::danger` or equivalent):

> **Required:** Fork [`awslabs/agent-plugins`](https://github.com/awslabs/agent-plugins) to your GitHub account. Do **not** point tasks at the upstream repo unless you have write access.

In **Register the repo in CDK**, add a matching `:::warning`:

> **Must match your fork:** Set `BLUEPRINT_REPO=your-username/agent-plugins` (or `-c blueprintRepo=…`) **before** build/deploy. The value must exactly match `--repo` in `bgagent submit`. Skipping this is the #1 cause of `REPO_NOT_ONBOARDED` and preflight failures.

#### 4. Post-deploy setup UX (align with #17)

Document the **current** manual steps clearly as a numbered checklist (store PAT, Cognito user, four stack outputs, `bgagent configure`, `bgagent login`). Add a "Coming soon" or tracked issue link for the ideal:

```bash
mise run setup # prompts PAT, email, password; reads outputs; configures CLI
```

Cross-link `bgagent admin invite-user` and `configure --from-bundle` for team onboarding.

#### 5. Static site information architecture revamp

Update `docs/astro.config.mjs` sidebar:

| New section | Content |
|-------------|---------|
| **Getting Started** (existing, improved) | Quick Start, Deployment Guide, **Setup checklist** (single linear page merging auth + CLI steps) |
| **Use Cases** (new) | e.g. "Fix a bug from a GitHub issue", "Iterate on a PR", "Trigger from CI webhook", "Linear label trigger" |
| **Tutorials** (new) | Step-by-step customization paths with expected outcomes |
| **Customizing** (expand) | **Onboard your own repository** — dedicated multi-step guide: *what* to change (`agent.ts` Blueprint, PAT, `BLUEPRINT_REPO`, Cedar policies), *why* (onboarding gate, preflight), *how* (fork → env var → deploy → submit) |

**Polish:**

- Consistent admonition styles for Required / Important / Tip
- "Next step" footer links at bottom of each getting-started page
- Landing page (`index`) with role-based paths: *I want to evaluate* / *I want to customize* / *I want to contribute*

#### 6. Acceptance criteria

- [ ] Quick Start opens with journey diagram + time estimate
- [ ] Prerequisites list GitHub profile, VPC permissions, and links to DEPLOYMENT_ROLES
- [ ] Fork + `BLUEPRINT_REPO` steps use danger/warning admonitions
- [ ] Single linear "first task" checklist (or `setup` command doc) reduces cross-page hopping
- [ ] Sidebar adds Use Cases + Tutorials; Customizing has "Onboard your own repo" guide
- [ ] `mise //docs:build` passes; Starlight mirrors committed

Start with docs/guides/QUICK_START.md, DEVELOPER_GUIDE.md, USER_GUIDE.md, and docs/astro.config.mjs, then compare the published Quick Start and issue #17. Trace the existing sync flow before restructuring the onboarding pages and sidebar. Done means the listed acceptance criteria are met, mise //docs:sync has updated the site mirror, and mise //docs:build passes.

Written by the indexing model from the issue text.