docs: Move the value collection documentation to Markdown

It needs to be in a separate page because it’s all macros and they have
no type/class associated with them.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3037

docs/reference/gobject/value-collection.md

@@ -0,0 +1,56 @@
+Title: Value Collection
+SPDX-License-Identifier: LGPL-2.1-or-later
+SPDX-FileCopyrightText: 2005 Matthias Clasen
+
+# Value Collection
+
+GLib provides a set of macros for the varargs parsing support needed
+in variadic GObject functions such as [ctor@GObject.Object.new] or
+[method@GObject.Object.set]
+
+They currently support the collection of integral types, floating point 
+types and pointers.
+
+## Macros
+
+`G_VALUE_COLLECT_INIT(value, _value_type, var_args, flags, __error)`
+
+:   Collects a variable argument value from a `va_list`.
+
+    We have to implement the varargs collection as a macro, because on some
+    systems `va_list` variables cannot be passed by reference.
+
+    Since: 2.24
+
+`G_VALUE_COLLECT_INIT2(value, g_vci_vtab, _value_type, var_args, flags, __error)`
+
+:   A variant of `G_VALUE_COLLECT_INIT` that provides the [struct@GObject.TypeValueTable]
+    to the caller.
+
+    Since: 2.74
+
+`G_VALUE_COLLECT(value, var_args, flags, __error)`
+
+:   Collects a variable argument value from a `va_list`.
+
+    We have to implement the varargs collection as a macro, because on some systems
+    `va_list` variables cannot be passed by reference.
+
+    Note: If you are creating the `value argument` just before calling this macro,
+    you should use the `G_VALUE_COLLECT_INIT` variant and pass the uninitialized
+    `GValue`. That variant is faster than `G_VALUE_COLLECT`.
+
+`G_VALUE_COLLECT_SKIP(_value_type, var_args)`
+
+:   Skip an argument of type `_value_type` from `var_args`.
+
+`G_VALUE_LCOPY(value, var_args, flags, __error)`
+
+:   Stores a value’s value into one or more argument locations from a `va_list`.
+
+    This is the inverse of G_VALUE_COLLECT().
+
+`G_VALUE_COLLECT_FORMAT_MAX_LENGTH`
+
+:   The maximal number of [type@GObject.TypeCValue]s which can be collected for a 
+    single `GValue`.