Create user guide documentation

[heitorlessa]

Document what's available for maintainers looking to use the code in here.

• GitHub client
• Filtering results
• Building key Markdown components from objects
• Available reports (and models)
• Running workflows across repositories
• Testing framework

[dreamorosi]

I would put this on hold until we have moved the codebase to TypeScript, at which point we can use TypeDoc and start with API docs generate from the extensive comments & annotations you already added.

[dreamorosi]

Just saw your comment here - I think I misunderstood the scope of this issue and API docs will in fact be addressed later on.