Adds defend.network under the Security category.

A free, no-auth JSON API of recently exploited CVEs, each enriched with CVSS, EPSS, and CISA KEV status (with due dates) and an exploitation-state field. Updated daily (~04:00 UTC), published under CC BY 4.0, with CORS enabled and curl/Python examples in the docs.

Per CONTRIBUTING, I added collection/defend-network.yaml and ran npm run build; the generated README.md entry and pages/defend-network/README.md are included. The README is not edited by hand.

Checklist

• YAML file validates (npm run validate reports no errors for defend-network.yaml)
• Slug matches filename (defend-network)
• At least one category selected from categories.yaml (Security)
• Links are reachable
• Entry does not duplicate an existing one
• npm run build ran successfully

Note: CI on main is currently red independently of this change — npm run validate exits non-zero due to ~564 pre-existing logo: null schema errors in other collection files, and the test suites fail to import because they reference ../src/*.js while the sources now live in cli/. This PR adds a single new, schema-valid entry and does not touch those files.