Developer Documentation

[danfunk]

In support of Bounties - assure we have documentation on our read-the-docs site to help people get started with development. Provide the following:

• [ ] Add a top level "Developers" section to the documentation
• [ ] Create a general on-boarding guide (how to set up a dev environment)
• [ ] Create an Architecture guide to explain the major components of the application - the library, Front-end, BPMN.io etc...
• [ ] Create an Extensions guide for how to create new extensions
• [ ] Create a Connectors guide for how to build connectors
• [ ] Create a Developers FAQ - move any FAQ items from our main page that are about software development and place them here.

[danfunk]

@harmeet-status - please verify that the sprint / priority / description are all ok on this new ticket.