Add links to Pangeo forum and Cookbooks org

[r-ford]

Discussed in ProjectPythia/projectpythia.github.io#267

[r-ford]

Since I don't have permissions here to add reviewers, @ProjectPythia/infrastructure

[brian-rose]

Thanks @r-ford for tackling this.

I don't think will quite do the trick though. GitHub looks for a profile/README.md file to display as organization "home page".
We did this in our other organization over at https://github.com/ProjectPythiaCookbooks/.github

See https://docs.github.com/en/organizations/collaborating-with-groups-in-organizations/customizing-your-organizations-profile#adding-a-public-organization-profile-readme

[r-ford]

@brian-rose Thanks, I just opened a new PR that should work