Display fully qualified class/namespace name as page header

Summary:
It's the most important information to see quickly what the page is about.
Using the fully qualified name contains the knowledge which namespace contains a
class/another namespace required which is helpful if one wants to use it in their
own code.

BUG: 406588

Test Plan:
Class:

Namespace:

Class inside namespace:

Nested namespace:

Class inside nested namespace:

Reviewers: ochurlaud

Subscribers: ngraham, aacid, jucato, kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D26197