document auditing Rust crates via Cargo Vet

[cfm]

Status

Ready for review

Description of Changes

Minimal documentation for starting to audit Rust crates via Cargo Vet, based on my experience in freedomofpress/securedrop#6999.

Testing

Visual review.

Release

No release considerations.

Checklist (Optional)

• [x] Doc linting (make docs-lint) passed locally
• [x] Doc link linting (make docs-linkcheck) passed
• [x] You have previewed (make docs) docs at http://localhost:8000

[legoktm]

Thanks for kicking this off @cfm, ✔️ for your part, if you want to review my additions then I'm happy to merge :)

[cfm]

I've applied a few nits and documented the policy we adopted in freedomofpress/securedrop#6999. Back to you, @legoktm!