[CT-902] CLI (Top level API) is automatically documented

[iknox-fa]

User Story:
As a user I would like to have accurate and easy to read documentation for how to use dbt-core as a library.

Acceptance Criteria:
An auto-documentation product (probably Sphinx) is set up and documents the top-level api. This runs as a GHA.

[leahwicz]

Let's make sure we account for accurate help text for commands which is referenced in this issue:
#5319

[iknox-fa]

FYI-- I think there's been some confusion vis-a-vis the documentation. This ticket is to generate sphinx docs as a reference, it doesn't refer to help text at all.

That said, after looking at #5319 we don't have to worry about it. The behavior described there is the default behavior for Click CLIs so we get that for "free"