Chore: migrate `onBrokenMarkdownLinks` to `siteConfig.markdown.hooks` (Docusaurus v4 deprecation)

[mmcdermott]

`npm run build` emits:

```
[WARNING] The `siteConfig.onBrokenMarkdownLinks` config option is deprecated and will be removed in Docusaurus v4.
Please migrate and move this option to `siteConfig.markdown.hooks.onBrokenMarkdownLinks` instead.
```

`docusaurus.config.ts` line 26 sets `onBrokenMarkdownLinks: 'warn'` at the top level. Move to `markdown.hooks.onBrokenMarkdownLinks` to be ready for the v4 bump.