Migrate module documentation from old book style

Problem/Motivation

The documentation for this module is in the old book style documentation.

Setting Major because the book style is deprecated.

Steps to reproduce

Proposed resolution

Migrate the docs to the contributed module guide → .

Remaining tasks

Go to https://www.drupal.org/node/2621770 → and follow the instructions
Make sure the documentation link of the project page is correct.

User interface changes

API changes

Data model changes