docs: clarify project structure and skip full CI for docs - #14

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, gitleaks/gitleaks-action@dcedce43c6f43de0b836d1fe38946645c9c638dc, rustsec/audit-check@69366f33c96575abad1ee0dba8212993eecbe998. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/