Add Data Streams self-service sign-up documentation

[j0sh-cl]

Summary

• Adds a new sign-up.mdx page under data-streams/ covering the full self-service portal sign-up flow
• Documents account creation, HMAC credential generation (one-time view, two-service/one-username model), stream catalog browsing, Stripe payment, and feed access
• Includes subscription terms, cancellation process (manual review flow), and custom plan contact link
• Adds the page to the sidebar under Chainlink Data Streams, between Overview and Developer Responsibilities

Test plan

• Page renders correctly at /data-streams/sign-up
• Sidebar entry appears and links correctly
• All internal links resolve (/data-streams/tutorials/go-sdk-fetch, /data-streams/reference/candlestick-api, /data-streams/billing, etc.)
• Aside components render as expected

[vercel]

@j0sh-cl is attempting to deploy a commit to the Chainlink Labs Team on Vercel.

A member of the Team first needs to authorize it.

[j0sh-cl]

Do not merge this PR until June 3rd or I give final confirmation