Schema ​

Schema and Models and Types define the data structures of an application.

Documenting ​

Document APIs

Try Open API (previously known as Swagger).

Or just text.

The best way to ensure that your API documentation is current and accurate is to embed it within your API implementation and then generate the documentation using literate programming techniques, or a framework such as http://apidocjs.com/, http://swagger.io/, or http://raml.org/index.html.

via: https://apiguide.readthedocs.io/en/latest/build_and_publish/documentation.html Document your API — API Design Guide 0.1 documentation

https://swagger.io/
The Best APIs are Built with Swagger Tools | Swagger
https://swagger.io/resources/open-api/
API Resources | Swagger
https://swagger.io/docs/specification/about/
About Swagger Specification | Documentation | Swagger | Swagger
http://editor.swagger.io/?_ga=2.216992021.268614673.1581966607-50034279.1581346793
Swagger Editor
https://raml.org/index.html
Welcome | RAML

Entity Relationship Diagrams ​

This is a commercial solution, but seems to be the best tool for the job:

https://duckduckgo.com/?t=ffab&q=lucid+charts&ia=web
lucid charts at DuckDuckGo
https://www.lucidchart.com/pages/
Intelligent Diagramming | Lucidchart
https://www.lucidchart.com/pages/product
Lucidchart overview | Lucidchart
https://lucid.app/pricing/lucidchart#/pricing
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