Add 2026 Training Week and update slides links - #212

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Unexpected input(s) 'node-version', valid inputs are ['repository', 'ref', 'token', 'ssh-key', 'ssh-known-hosts', 'ssh-strict', 'persist-credentials', 'path', 'clean', 'sparse-checkout', 'sparse-checkout-cone-mode', 'fetch-depth', 'fetch-tags', 'lfs', 'submodules', 'set-safe-directory', 'github-server-url', 'allow-unsafe-pr-checkout']