app,ui: move instructions link to package app documentation

We're about the split up and finally delete package ui. Move the
instructions link to the second most "top-level" package in Gio,
package app.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/doc.go

@@ -4,6 +4,8 @@
 Package app provides a platform-independent interface to operating system
 functionality for running graphical user interfaces.
 
+See https://gioui.org for instructions to set up and run Gio programs.
+
 Windows
 
 Create a new Window by calling NewWindow. On mobile platforms or when Gio