doc: add skeleton documentation

This adds API documentation, generated at compile time.

doc/reference/gnome-calendar.hierarchy

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+GObject
+  ECalDataModel
+  GApplication
+    GtkApplication
+      GcalApplication
+  GInitiallyUnowned
+    GtkWidget
+      GtkContainer
+        GtkBin
+          GcalDateChooserDay
+          GcalDateChooser
+          GtkWindow
+            GtkDialog
+              GcalEditDialog
+              GcalSourceDialog
+            GtkApplicationWindow
+              GcalWindow
+          GtkPopover
+            GcalQuickAddPopover
+            GcalSearchView
+          GtkButton
+            GtkToggleButton
+              GtkMenuButton
+                GcalTimeSelector
+        GcalSubscriberView
+          GcalMonthView
+        GtkBox
+          GcalMultiChoice
+          GcalWeekView
+          GcalYearView
+        GcalWeekGrid
+        GtkGrid
+          GcalWeekHeader
+        GtkMenuShell
+          GtkMenu
+      GtkEntry
+        GcalDateSelector
+      GcalEventWidget
+  GcalEvent
+  GcalManager
+  GDBusProxy
+    GcalShellSearchProvider2Proxy
+  GDBusInterfaceSkeleton
+    GcalShellSearchProvider2Skeleton
+  GcalShellSearchProvider
+  ECalClientView
+  GApplicationCommandLine
+  GMenuModel
+  AtkObject
+    GtkAccessible
+      GtkWidgetAccessible
+        GtkContainerAccessible
+          GtkWindowAccessible
+          GtkPopoverAccessible
+          GtkButtonAccessible
+            GtkToggleButtonAccessible
+              GtkMenuButtonAccessible
+        GtkEntryAccessible
+  GtkStyle
+  GdkWindow
+  GdkDragContext
+  GtkTooltip
+  GdkScreen
+  GdkDisplayManager
+  GdkDisplay
+  GtkEntryBuffer
+  GdkPixbuf
+  GtkEntryCompletion
+  GZlibDecompressor
+  GFileInfo
+  ECalComponent
+  ESource
+  GSettings
+  GDBusMethodInvocation
+  GDBusConnection
+  ESourceExtension
+    ESourceLocal
+  GtkAction
+GInterface
+  GTypePlugin
+  ECalDataModelSubscriber
+  GActionGroup
+  GActionMap
+  AtkImplementorIface
+  GtkBuildable
+  GtkEditable
+  GtkCellEditable
+  GInitable
+  GtkOrientable
+  GcalView
+  GcalShellSearchProvider2
+  GDBusInterface
+  GAsyncInitable
+  GtkActionable
+  GtkActivatable
+  AtkComponent
+  GIcon
+  GLoadableIcon
+  GtkCellLayout
+  AtkEditableText
+  AtkText
+  AtkAction
+  AtkWindow
+  GConverter
+  GProxyResolver
+  AtkImage
+GEnum
+  EventEditableProperty
+  GcalWindowViewType
+  GtkAlign
+  GtkStateType
+  GtkTextDirection
+  GtkDirectionType
+  GtkDragResult
+  GtkWidgetHelpType
+  GtkResizeMode
+  GtkShadowType
+  GtkImageType
+  GtkInputPurpose
+  GtkMovementStep
+  GtkDeleteType
+  GtkEntryIconPosition
+  GtkWindowType
+  GtkWindowPosition
+  GdkWindowTypeHint
+  GdkGravity
+  GZlibCompressorFormat
+  GtkOrientation
+  GtkBaselinePosition
+  GtkPackType
+  GtkPositionType
+  GtkPopoverConstraint
+  GBusType
+  GtkReliefStyle
+  GtkArrowType
+GBoxed
+  GValueArray
+  GcalRangeTree
+  icaltimetype
+  GError
+  GVariantDict
+  GdkRectangle
+  CairoRectangleInt
+  CairoContext
+  GdkEvent
+  GtkSelectionData
+  GtkRequisition
+  GtkBorder
+  GdkColor
+  GDateTime
+  PangoAttrList
+  PangoTabArray
+  GdkRGBA
+  GTimeZone
+  GStrv
+  GDBusInterfaceInfo