Link to external catalogs

[cisaacstern]

430 removes the (unmaintained) catalog page.

Going forward, we may want to link to external catalogs that contain data generated by Pangeo Forge, e.g.:

• https://leap-data-catalog.vercel.app
  • See https://github.com/leap-stc/data-management/pull/74
• https://github.com/leap-stc/cmip6-leap-feedstock/tree/main#how-to-access-the-newly-uploaded-data

Should we also catalog feedstock repositories somehow?