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/initialization.md

@@ -0,0 +1,34 @@
+Title: Initialization
+Slug: initialization
+
+# Initialization
+
+Before using libfeedback, it must be initialized. To use the library
+call [func@Lfb.init]() with the id of your application (usually
+the desktop file name without the .desktop extension). For that you
+include the `libfeedback.h` header. Since the API is considered
+unstable at this point in time you need to acknowledge this by
+definining `LIBFEEDBACK_USE_UNSTABLE_API`:
+
+```c
+  #define LIBFEEDBACK_USE_UNSTABLE_API
+  #include <libfeedback.h>
+
+  int main(void)
+  {
+     g_autoptr (GError) *err = NULL;
+     if (lfb_init ("com.example.appid", &err)) {
+       g_error ("%s", err->message);
+     }
+     ...
+     lfb_uninit ();
+     return 0;
+  }
+```
+
+You can also acknowledge this with the definition option of your C
+compiler, like `-DFEEDBACK_USE_UNSTABLE_API`.
+
+After initializing the library you can trigger feedback using
+[class@Lfb.Event] objects.  When your application finishes call
+[func@Lfb.uninit]() to free any resources: