docs: expand pre-artifact attestation binding details

[mintlify]

Documents how --commit works for pre-artifact attestations (e.g. kosli attest generic, junit, custom):

• Adds a detailed worked example to Tutorials → Reporting a custom attestation → Attest before the artifact exists, explaining that Kosli stores the attestation as pending against (template artifact name, git commit) and binds it to the fingerprint when the artifact is later reported. Covers 8 subtleties: match key, template name requirement, same-commit-on-both-sides, real-git-repo requirement, multiple-matching-artifacts behavior, ordering independence, template-less flows, and that --commit is only required for the pre-artifact case.
• Adds a short callout to Getting started → Attestations on the backend.unit-tests step summarizing the same binding model and linking out to the tutorial.

The kosli attest generic / kosli attest custom CLI synopsis pages are auto-generated, so the prose lives in the tutorial and getting-started pages instead.

[mintlify]

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