The GOV.UK Frontend details component macro
wraps its `<summary>` text in a `<span>`.
We put a `<h3>` in the `<summary>` (actually valid
use, based on the spec) so this breaks when the
`<span>` wraps it.
This converts the existing `<details>` tag to use
all the class names the macro creates, but with
all the `<summary>` contents in the `<h3>`.
Also adds font-smoothing to the messages on the
API page. This was previously set globally for
all fonts in the GOV.UK Template CSS but is now
just set for the New Transport 'nta' font.
Included because the messages use the monospace
font so don't have it by default.