Index and render index / indexentry fields

[marlo-longley]

Duke has a solution for this locally that we can reference.

This ticket is broken out of #898

[seanaery]

Our solution at Duke (which we would be happy to PR to core) is to turn these elements into HTML tables. See format_indexes & format_indexentries...

https://gitlab.oit.duke.edu/dul-its/dul-arclight/-/blob/main/app/helpers/ead_format_helpers.rb#L38
https://gitlab.oit.duke.edu/dul-its/dul-arclight/-/blob/main/app/helpers/ead_format_helpers.rb#L77-102

Examples in action:
https://archives.lib.duke.edu/catalog/jwtnewsletters#other-indexes
https://archives.lib.duke.edu/catalog/gilpin#other-indexes

[seanaery]

EAD2002 docs re:
<index>: https://www.loc.gov/ead/tglib/elements/index-element.html
<indexentry>: https://www.loc.gov/ead/tglib/elements/indexentry.html