QDoc: render template teletype as <code>

The Template Generator's HTML partials renders QDoc's `\tt` command
as `<tt>`, an element HTML5 removed. The same partials render `\c`
as `<code>`, so the two commands produces structurally different
output for visually similar text.

This change makes QDoc render Teletype with the same `<code>` element
the partials use for Code.

Pick-to: 6.12
Change-Id: I6311dd8067ffcc8dcb5ef05354929fb9f393fbba
Reviewed-by: David Boddie <david.boddie@qt.io>