Add headings to overview

doc/variant2/overview.adoc

@@ -9,7 +9,9 @@ http://www.boost.org/LICENSE_1_0.txt
 
 [#overview]
 # Overview
-:idprefix:
+:idprefix: overview_
+
+## Description
 
 This library implements a type-safe discriminated/tagged union type,
 `variant<T...>`, that is API-compatible with the {cpp}17 Standard's
@@ -39,6 +41,8 @@ struct V
 };
 ```
 
+## Usage Examples
+
 Variants can be used to represent dynamically-typed values. A configuration
 file of the form
 
@@ -167,6 +171,8 @@ int main()
 }
 ```
 
+## Construction and Assignment
+
 If we look at the
 
 ```
@@ -194,6 +200,8 @@ variant::variant(float x);
 and the standard overload resolution rules are used to pick the one that will
 be used. So `variant<int, float>((short)1)` will hold an `int`.
 
+## Inspecting the Value
+
 Putting values into a `variant` is easy, but taking them out is necessarily a
 bit more convoluted. It's not possible for `variant<int, float>` to define a
 member function `get() const`, because such a function will need its return
@@ -265,6 +273,8 @@ void f( variant<int, float> const& v )
 }
 ```
 
+## Visitation
+
 Last but not least, there's `visit`. `visit(f, v)` calls the a function object
 `f` with the value contained in the `variant` `v` and returns the result. When
 `v` is `variant<int, float>`, it will call `f` with either an `int` or a
@@ -311,6 +321,8 @@ void g( variant<int, float> const& v )
 `f(x1, x2)`, where `x1` is the value contained in `v1` and `x2` is the value
 in `v2`.
 
+## Default Construction
+
 The default constructor of `variant` value-initializes the first type in
 the list. `variant<int, float>{}` holds `0` (of type `int`), and
 `variant<float, int>{}` holds `0.0f`.