message: doc(API): Provide an overview of the different sections
diff: @@ -2,3 +2,13 @@ title: API
  ---
  This documentation provides more detailed information about the *itk.js* API.
  +
  +The **Input/Output** section describes functions that facilitate reading and
  +writing data from the local filesystem, if working in Node.js, or from native
  +browser data types, encountered when working in the web browser.
  +
  +The **Processing Pipelines** section describes dow to execute processing
  +pipelines written as C/C++ command line executables in Node.js or the browser.
  +
  +The **Data Structures** section describes data structures produced and consumed
  +by both the *Input/Output* and *Processing Pipelines* functions.
  