doc: move Usage and Example to same header level

Having Example under Usage in synopsis.md is misleading. That suggests
that the examples will be examples of the CLI usage, but the example
section is mostly about writing a simple web server. Ideally, the Usage
section should be moved to cli.md and the Example section should
constitute a Getting Started or Quick Start page. But for now, make them
equals under a combined header so that the Table of Contents and the
header/layout of the page is not confusing or misleading.

PR-URL: #28570
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/synopsis.md

@@ -1,4 +1,6 @@
-# Usage
+# Usage & Example
+
+## Usage
 
 <!--introduced_in=v0.10.0-->
 <!--type=misc-->