Document all ways / places where an extension can extend Platform

[Sebastian-ubs]

User Story
As a extension developer, I want to know which parts of Platform I can extend so that I can understand the possibilities and work with them or request other extension points.

Description
Questions like "can an extension add a component to the main toolbar" or "can an extension replace an existing extension" lead to a need for documentation. Add a place where all possibilities are listed how to extend Platform - together with links and/or examples.
Incomplete list of examples

• main menu
• tab menu
• context menu
• main toolbar?
• tab toolbar
• settings
• services? data model? ...
• project types?
• ...
• additional projects / resources (e.g. online)
• another way to open a project / resource (editor / viewer), e.g.
  • dictionary
• another way to edit data (not the text) of a project / resource, e.g.
  • different biblical terms tool
  • different notes viewer
  • different spell checker

Implementation idea
Add a wiki page to the extensions wiki

Testing ideas

[Sebastian-ubs]

partially depends on

• https://github.com/paranext/ux/issues/167
• https://github.com/paranext/ux/issues/203

related

• https://github.com/paranext/ux/issues/192
• https://github.com/paranext/ux/issues/181