Update billing alerts docs for new standalone page

[matt423]

Summary

• Update billing alerts URL from /accounts/:id/package#billing-alerts-section to /accounts/:id/billing_alerts
• Rewrite setup steps to reflect the new modal-based create flow and sidebar navigation
• Clarify recipient selection behaviour for owners vs billing users

Related: https://github.com/ably/website/pull/7910

Test plan

• Verify the billing alerts link resolves correctly once website PR is merged
• Review the updated steps against the new UI

[coderabbitai]

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