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.

README.md

@@ -104,7 +104,7 @@ mem0 add "I am using mem0"
 mem0 search "am I using mem0"
 ```
 
-The human owner can claim the account later with `mem0 init --email <their-email>` — same key, memories preserved.
+The human owner can claim the account later with `mem0 init --email <their-email>` — same key, memories preserved. Full guide: [Sign up as an agent](https://docs.mem0.ai/platform/agent-signup).
 
 | | Library | Self-Hosted Server | Cloud Platform |
 |---|---------|-------------------|----------------|