Document the different ways orangelight communicates with figgy to retrieve the viewer.

[christinach]

I started drafting https://docs.google.com/document/d/1MiltehCL79gspKp_wd4k7Bqo3VXguV6aIN5Z5VoGK24/edit

related issues:
pulibrary/bibdata#505 Remove figgy ark cache?
pulibrary/bibdata#1720 Figgy arks resolving to voyager ids instead of Alma ids.
pulibrary/bibdata#1724 Indexing 856 content on records without anh holdings or portfolios.
pulibrary/bibdata#770 Figgy ARK overrides shouldn't replace the 856 label with 'Digital content' if it's not a link.
pulibrary/bibdata#1848 Update fragment to be viewer-container
#2147 Default book/manuscript thumbnail misses the icon to navigate to the viewer.

[kevinreiss]

Still need to add documentation about Figgy-OL communications specific to CDL.

[christinach]

closed by #3144