Polish and expose auto-generated Workflow step documentation

The first work for JENKINS-26126 added the internals (and a proof of concept HTML output) for introspecting Workflow steps and returning information about them, such as arguments, help docs, etc. That HTML needs to be cleaned up into a presentable form and, along with the GDSL and DSLD that'll come from JENKINS-26126, it needs to be exposed like the /cli endpoint on a Jenkins master with Workflow installed.