Document new STAC Manager

[j08lue]

The new architecture for STAC Manager is taking shape and the new app has feature parity with the original STAC Admin.

• Rewrite application with form plugin system and new design #18

Before we merge the re-implementation, a few things should get documented (list from @danielfdsilva), so we deliver a usable and understandable solution:

• What is a plugin and what is it used for?
• Creating a plugin: The schema (describe all the options)
• Creating a plugin: how the schema maps to default widgets / how to set a custom widget
• Creating a plugin: Enter/exit data
• Creating a plugin: Utility functions
• Widgets: Existent ones
• Widgets: Creating custom widgets
• Client Form: Requirements to implement
• Client Form: Creating the form (useCollectionPlugins, PluginBox, WidgetRenderer)
• Client Form: Validation and data submission

[j08lue]

The new docs in https://github.com/developmentseed/stac-manager/tree/stac-manager/docs seem to cover all the mentioned topics and look well laid out. I cannot speak to how user-friendly they are for developers or contributors, but definitely work for closing this as completed on the PR is merged.