split out repository to separate code from content; make theme installable as a hugo module

[rlskoeser]

https://docs.github.com/en/get-started/using-git/splitting-a-subfolder-out-into-a-new-repository

• split current repo into one for the hugo site and one for the theme+code
• make the theme repo usable as a hugo module
• configure the site repo to use the theme
• transfer the hostname to the new repo

[rlskoeser]

completed

• code only in this repo; installable as a hugo module
• content in https://github.com/Princeton-CDH/lenape-timetree-content ; configured to deploy the site from this repo
• read-only archive of combined repo https://github.com/Princeton-CDH/lenape-timetree-archive