Developer Guide: Fix not parsed headers (#12332)
The following two snippets can be found in the current Developer Guide as rendered in HTML format:
`+++ An Example Manifest File +++`
`++ Plugins and Drivers ++`
These have been around for quite some time.
It seems quite obvious they should be rendered as headers, respectively:
`4.2.2. An Example Manifest File`
`4.3. Plugins and Drivers`
