info worker: use kdoctools' kde-docs.css instead of kio_docfilter.css

kio_docfilter.css got out of sync with with kdoctools, e.g. for the header. Using the same CSS as the application manuals also results in visual consistency e.g. in khelpcenter.

After:

Before:

@ltoscano

See !346 (merged) for similar work for the man worker