Doc: Drop the use of QT_PLUGIN_PATH in documentation config

This caused build issues in certain configurations. We don't actually
need it, as we can rely on the base Qt include path instead.

Also move the custom module header under src/webengine/doc and rename
it.

Task-number: QTBUG-67342
Change-Id: Id5e809202ddaa285bfcc341a36b0b3d5a7ade436
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>