Transfer Pipeable Operator Documentation from ‘old docs’

In the old docs (‘/doc’) there is a file called ‘pipeable-operator.md’. This needs to be transfered to the new docs (‘docs_app/content/guide/v6’). The content itself should be rxjs 6 related, so this might don’t need any fixes. But one could consider improving styling and maybe the transfer results in broken links.
Maybe some of the examples should be changed to Typescript