Add a config entity to allow admin to enter per-path overviews

Problem/Motivation

We allow the admin to enter a general overview, however some overview text per-endpoint is a reasonable expectation.

Proposed resolution

Add a config entity with a 'path' machine-name and a text overview field. Load the config entity in the detail callback (api/doc/{path}) and display the text.

Remaining tasks

Code
Tests
Review

User interface changes

Need to add an overview page and UI to manage config entities.

API changes

None