Support for `docs/config.json`

[pvande]

Proposal: Add support for docs/config.json as a valid configuration endpoint, in addition to docs.json and docs.yaml.

Rationale: Allows all documentation-related files to live under docs, providing better encapsulation. This also allows generated documentation to remain self-contained, which can be an advantage to that build process.

Potential Drawbacks: Adding another configuration file location weakens the convention overall, and potentially opens the doors to additional conversations in the future (e.g. docs/config.yaml?). Adds an additional API test when looking for configuration.