docs: replace pre-alpha notice with waitlist prompt

[posthog]

Summary

• Replaces the README's pre-alpha "not production-ready" notice with a prompt to join the waitlist at https://posthog.com/code (Discord link kept as a secondary option).

Test plan

• Render the README on GitHub and verify the new callout displays correctly with both links working.

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Created with PostHog Code

[greptile-apps]

_{Reviews (1): Last reviewed commit: "docs: replace pre-alpha notice with wait..." | Re-trigger Greptile}