docs: update Inkeep managed platform link

[JSap0914]

Fixes #3429.

Summary

• replace the stale Inkeep Cloud waitlist link in the README with the current Get started page
• keep the adjacent Enterprise link unchanged

Verification

• curl -L https://inkeep.com/cloud-waitlist returns 404
• curl -L https://inkeep.com/get-started returns 200
• curl -L https://inkeep.com/enterprise returns 200 and redirects to /cx-agents
• git diff --check HEAD~1..HEAD -- README.md

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