Generate manpage from configuration options

In https://github.com/jupyter/jupyter_server/issues/320, @SnarkBoojum submitted a manpage for Jupyter Server that largely encoded the results from `--help`. We realized that it would make more sense to have an automatically generated manpage at the traitlet level from the configurable traits. This issue is about adding a `--generate-manpage` option, or perhaps a separate manpage generation command, that does something similar to `-h`, but generates the output as a manpage.

Start by tracing how configurable traits produce the existing `-h` output and how a command-line option or separate command would fit at the traitlet level. Define whether `--generate-manpage` or a separate command is the intended interface, then verify that the generated output captures the configurable options in manpage form.

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