Fetch icons a stale cached set lacks, and scan Markdown - #3

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

"xref: [exclude: ...]" in your mix.exs file is deprecated, instead use: "elixirc_options: [no_warn_undefined: ...]"