chore: update JIRA account instructions to use ASF self-serve portal

[Croway]

Summary

• ASF JIRA no longer allows self-registration via the "Log In" link
• Updated the contributing guide to point users to the ASF self-serve portal at https://selfserve.apache.org/jira-account.html

Claude Code on behalf of Federico Mariani

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🧪 CI tested the following changed modules:

• docs

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