Index external tutorials

[jeffjennings]

Potential sources:

• radio-astro-tools
• STScI
• Caltech/IPAC–IRSA
• CDS-astro
• Python for Astronomers
• codeastro
• DESI
• Toronto workshop content (Nathaniel), e.g. Coding the Cosmos
• Workshop tutorials as a book - each workshop is in its own branch, so there’s probably not anything unique in past-astropy-workshops

Implementation:

• do they need to add metadata files for algolia to index them?
• set up repo for indexing external content - see indexjupyterbookpage.py