Switch library documentation to gi-docgen

Switch from gtk-doc to gi-docgen for the library documentation and
adjust the source files accordingly.

We don't bother about the daemon doc headers as they're not built into
any docs atm.

Closes: #56

doc/overview.md

@@ -0,0 +1,23 @@
+Title: Overview
+Slug: Overview
+
+# Libfeedback Introduction
+
+The purpose of libfeedback is to make it simple to supply audible,
+haptic and visual feedback to the user. This is done by
+notifying a feedback_daemon over DBus that a certain event
+happened. The daemon then selects the appropriate feedbacks (such
+as a buzzing haptic motor or playing an audio file) based on the
+user session's current feedback profile, feedback theme and
+available hardware.
+
+Events are identified by strings like `message-new-sms` or
+`message-new-sms`. The available event names are described in the
+[Event naming specification](Event-naming-spec-0.0.0.html).
+
+Libfeedback provides synchronous and asynchronous APIs to trigger and
+stop feedback for these events and is usable from other languages than
+C by GObject introspection.
+
+See [Compiling with libfeedback](build-howto.html) on how to include
+libfeedback in your project.