[Standard Documentation View] Set the correct mime type for job replies

The mime type was always set to text/html resulting in warnings like "Resource
interpreted as Stylesheet but transferred with MIME type text/html: [...] "
displayed in attached developer tools. This prevented it from being used
by webengine resulting in pages without style information. I believe this
behavior change was introduced with Qt 5.14 since I first saw this after
upgrading. Also register the qthelp scheme to avoid a similiar situation in the
future as suggested by the warning "Please register the custom scheme 'qthelp'
via QWebEngineUrlScheme::registerScheme() before installing the custom scheme
handler".