Improve API Reference docs

[jonatasbaldin]

The API Reference docs is auto generated from the source code (comments on the Types).

We need an overall look on that and improve the comments accordingly.

We also need to add a Table of Contents.

[jonatasbaldin]

The main challenge will be to transform the API Reference templates into markdown, since Doscy can't build a TOC for HTML content.

[mhoffm-aiven]

TOC added in #159