docs(a2a): document A2A agent on /api-docs/ + add A2A quickstart

[jung-thomas]

Why

The A2A (Agent2Agent) endpoint is a real first-party surface — public Agent Card at `/.well-known/agent-card.json`, JSON-RPC `POST /a2a`, five skills, ORD-registered — but it was only discoverable via `.well-known` and the ORD descriptor. Nothing on the human-facing `/api-docs/` landing page mentioned it, and there was no quickstart like the MCP one.

What

• `hugo/content/api-docs/_index.md` — new A2A agent section (sibling to MCP server): what it is + how it differs from MCP, public Agent Card discovery, JSON-RPC transport, the five-skill table, the `Tutorial.MCP` client-credentials auth split (public card vs. authenticated calls), an inline `message/send` curl, and a link to the quickstart. Frontmatter `description` updated to mention A2A.
• `docs/end-users/a2a-quickstart.md` (new) — mirrors `mcp-quickstart.md`: public discovery → internal/partner client-credentials (grounded in the app's XSUAA `cf env` credentials + a note that granting the scope to a separate consumer is a BTP-admin step), `message/send` / `message/stream` (SSE) / `tasks/get` examples, JSON-RPC error-code table (grounded against `srv/lib/a2a/rpc-router.js`), and the `/admin-ui/#joule` config panel.
• `docs/.vitepress/config.ts` — register the new quickstart in the End Users sidebar (required by `check-docs-sidebar.cjs`).

Docs only — no changes to `agent-card.js`, `rpc-router.js`, or ORD annotations. Deliberately did not invent Joule Studio A2A UI click-paths.

Verification

• `node scripts/check-docs-sidebar.cjs` → ok (133 pages, 142 links, includes the new page).
• `npx vitepress build docs` → build complete, no errors.
• Error-code table (`-32001`→401; `-32601/-32602/-32603`→200; 503 when disabled) verified against `srv/lib/a2a/rpc-router.js`.
• All referenced files confirmed present.