docs: dedicate /platform/agent-signup page; rework introduction + nav

Per launch feedback (Deshraj, Mragank, Rudraj on 2026-05-16):

- NEW: docs/platform/agent-signup.mdx — dedicated tutorial page. Owns
  the full Agent Mode story: 4-command demo, --agent-caller, mem0
  identify fallback, claim flow, rate limits, what's-next links.

- docs/docs.json: insert platform/agent-signup at position 2 in the
  Platform > Getting Started group (directly under platform/overview).

- docs/introduction.mdx: the homepage card formerly labeled "CLI" and
  linking to /platform/cli now points to /platform/agent-signup with
  the headline "Sign up as an agent" — the first big call-to-action.

- docs/platform/cli.mdx: stripped the Agent Mode promo block, the
  Plugin Sync section, the agent-related rows from the mem0 init flag
  table, and the entire mem0 identify subsection. Added a one-line
  wayfinding Tip + Note pointing readers to /platform/agent-signup.
  CLI page is now a clean reference for human dashboard users.

- docs/platform/quickstart.mdx: reverted the duplicate 4-step block
  added in the earlier commit. Replaced with a one-line Note pointing
  to /platform/agent-signup.

- README.md: closing pointer added to the existing Quickstart subtitle
  so the GitHub landing page lands readers in the docs.

docs/docs.json

@@ -40,6 +40,7 @@
                     "icon": "rocket",
                     "pages": [
                       "platform/overview",
+                      "platform/agent-signup",
                       "vibecoding",
                       "platform/mem0-mcp",
                       "platform/cli",