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.

首先追踪可配置 trait 如何生成现有的 `-h` 输出，以及命令行选项或独立命令如何在 traitlet 层级接入。确定预期的接口是 `--generate-manpage` 还是独立命令，然后验证生成的输出是否以 manpage 形式包含可配置选项。

由索引模型根据 Issue 内容生成。