docsy.dev: install link-cache from the npm registry

[chalin]

• Follow-up to docsy.dev: run link-check bins directly, not via npx #2686: switches docsy.dev's link-cache devDependency from the GitHub pin (github:chalin/link-cache#semver:^0.2.0) to the package's first npm registry release, published with the registry's strictest settings (trusted publishing, 2FA required, tokens disallowed).
• Registry installs are faster (no git clone) and immutable — a published version's tarball can't change, which matters here since lockfiles are gitignored.
• Verified: npm install resolves link-cache@0.3.0 from the registry, and the lychee-norm-cache / refcache bins run from node_modules/.bin.