docs(model-providers/morph): fix dead morphllm.com/dashboard link

[sanjibani]

Replaces https://morphllm.com/dashboard (404) with https://morphllm.com/sign-up (200) — the old dashboard URL was retired.

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Summary by cubic

Updated the Morph provider docs to replace the dead dashboard link with the working sign-up page. This prevents 404s and keeps the setup flow smooth for new users.

^{Written for commit c1a28f1. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 1 file

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