Add Fathom Analytics to docs site

Set up Fathom Analytics on the docs site (miren.md). The site is Docusaurus deployed via GitHub Pages from this repo.

Need to figure out:

• Fathom account/site setup
• How to inject the Fathom tracking snippet into the Docusaurus build (likely via scripts config in docusaurus.config.ts or a custom plugin)
• Ensure it works with the GitHub Pages deployment pipeline