[Iheanacho-ai]

Updates the docs for Omni v1.12.4 and Talos v1.15.0-alpha.1.

• Bumps the custom variables: omni_release to v1.12.4, omni_helm_chart_release to 2.12.4, and release_v1_15 to v1.15.0-alpha.1.
• Pins the reference generators to the new releases: talosctl v1.15.0-alpha.1 in the Makefile and omnictl v1.12.4 in the omni-cli-gen Dockerfile.
• Regenerates the Omni CLI and configuration references. The configuration reference is built from the v1.12.4 schema, not main.
• Regenerates the Talos v1.15 CLI and configuration references and adds the 7 new configuration pages to talos-v1.15.yaml.
• Regenerates the changelog.

[github-actions]

📋 Automated documentation review

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[sterlingkoch]

Merged as part of #867.