Improve the description of AppD, its role and value.

[kriswest]

Area of Issue

[x] App Directory

Issue Description:

The description of the AppD in the FDC3 website is poor and fails to properly outline its role in FDC3, intended use and ultimately its value as part of the standard, potentially leading to recent proposals to remove it from the standard (#551 #550).

The description should be improved to the point that an individual new to FDC3 can immediately understand its role and why they might want to use it (and/or encourage vendors they work with to use it).

The expected interactions between Desktop Agents and AppD should also be clarified as some readers have interpreted the presence intent configuration data in the appD as a requirement for intent resolution to involve queries to the appD during resolution or for the appD to control intent resolution, when it is actually only intended to provide data to support intent resolution, where implementing resolution is the province of the Desktop Agent implementation (#551).