Document CosmoDC2

[heather999]

• Add link to CosmoDC2 papge
• Replicate as appropriate the content of https://portal.nersc.gov/project/lsst/cosmoDC2/_README.html

[yymao]

A markdown version of that page can be found in https://portal.nersc.gov/project/lsst/cosmoDC2/_README.md.

• Step 1 can add conda (or just point to our new doc to installing GCRCatalogs)
• Step 3 need to be updated to use the new cosmoDC2 config.