docs: move the page closer to the standard template for documentation (…

…#398)

docs/index.rst

@@ -6,12 +6,20 @@
 Welcome to Craft Parts' documentation!
 ======================================
 
+Craft Parts is a Python package to support a family of tools that create
+standalone software packages for deployment on Linux-based systems.
+
 Craft Parts provides a mechanism to obtain data from different sources,
 process it in various ways, and prepare a filesystem subtree suitable for
 deployment. The components used in its project specification are called
-*parts*, which can be independently downloaded, built and installed, and
-also depend on each other in order to assemble the subtree containing the
-final artifacts.
+*parts*, which are independently downloaded, built and installed, but can
+also depend on each other.
+
+This package implements common functionality to prepare package data that
+would otherwise be duplicated in separate tools.
+
+Craft Parts is useful for implementers of packaging tools that share a
+similar view of how data should be processed and prepared for deployment.
 
 .. toctree::
    :maxdepth: 1