Add intro to API docs

[kelseiv]

Proposal:
Add preface or introductory material at the beginning of the swagger API docs.

Current behavior:
On the main API docs page, we don't find any overview content to get started.

Desired behavior:
Give folks an overview of the basics/where to start. Accepted request body formats, return response formats, response codes used, errors, how authentication works.

Use case:
This info is important for developers using our API.

cc @stephanie-engel @rhajek @influxdata/docs-team

[duanshuaimin]

Good idea