[searchableguy]

You can use something like forestry: https://forestry.io or any of the few alternatives for managing content for your static generator site.

[FrontAid]

Or have a look at https://frontaid.io/ if you want to store your content in a JSON file in your own Git repository.

[zapt02]

Is there something like Forestry (CMS with Git as datastore) but self-hosted and/or open-source?

[searchableguy]

https://github.com/GetPublii/Publii

https://www.netlifycms.org/

https://github.com/sanity-io/sanity

[sionillinois]

Yes, NetlifyCMS.