Adds documentation for GUSTO

Adds a comprehensive documentation page detailing how to use GUSTO.

The documentation covers topics such as:
- Getting started with the library
- Implementing a provider
- Implementing batching and continuations
- Scheduling cron jobs
- Implementing retry policies
- Testing the library

The documentation aims to provide users with a clear and concise guide on how to use GUSTO Field Kit to its full potential.

Author: VisualBean