docs: refresh README to match converged Nylas SDK style

[nbarraclough]

Summary

• Aligns the Ruby SDK README with the visual + structural conventions used across the official Nylas SDKs: centered HTML header with logo, badge row (gem version · codecov · downloads · MIT), dot-separated quick links, and emoji-prefixed section headers in the canonical order.
• Removes stale v3-beta and pre-v3 dashboard links; routes readers to developer.nylas.com instead of duplicating docs locally. Adds AI agents, Security, Reference, and Other Nylas SDKs sections.
• Adds a short Ruby-specific note in Usage explaining the tuple return convention ([data, request_id, next_cursor, headers] for list endpoints) and a pagination snippet using next_cursor + query_params: { page_token: cursor }. Verified against lib/nylas/resources/calendars.rb and lib/nylas/handler/api_operations.rb.
• Copies diagrams/nylas-logo.png into the repo so the header renders without hotlinking.

Test plan

• Render the README on the docs/readme-refresh branch on GitHub and confirm the centered header, badges, and quick-link row display correctly.
• Spot-check that all developer.nylas.com links resolve.
• Confirm no dashboard.nylas.com, /docs/v3-beta/, or info.nylas.com/apiv3betasignup references remain.
• Confirm the pagination snippet runs end-to-end against a real grant.

🤖 Generated with Claude Code

[qasim-nylas]

LGTM 👍