ci: bump pymdown-extensions from 10.21.3 to 11.0.1 in /docs - #26

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: contributor-assistant/github-action@v2.6.1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/