docs: add cosign keyless documentation

[akashsinghal]

Adds documentation for the new cosign keyless experience for trust policy

• Moves docs for previous keyless support to be under legacy section
• Creates a new keyless section explaining how keyless verification is configured
• updates Kubernetes config section to add new fields

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[akashsinghal]

TODO: add section showing what the verifier report looks like for keyed and keyless cosign verification