Docs as code: hashicorp/terraform-plugin-docs

Problem

Adding/changing resources or attributes requires that a contributor also update the docs, and there is no automated check for this. Inconsistencies continually come up. (For more, see "Docs" section of issue #427 (closed))

Suggestion

The Terraform SDK lets you document attributes in-code. We should switch to writing documentation in-code, and use the official hashicorp/terraform-plugin-docs generator/validator to generate the documentation markdown.