[Pulsar Client Tools]Support generate documentation of pulsar admin cli automatically (#5738)
### Motivation
Currently, the commands on the page http://pulsar.apache.org/docs/en/pulsar-admin/ are all manually added. when the document changes, there are often wrong contents. in order to solve this problem, we expect the commands on this page to be automatically generated. this pr is the first step. If it passes, we will add a new page on the website to show the automatically generated commands later.
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