[documentation] Auto-generate Kagami docs from clap

[0x009922]

Description

The documentation for Kagami (in its README) is outdated. It will be even more outdated after the merge of #3475.

Your suggestions

Use clap-markdown or similar tool.

[0x009922]

After a discussion with the team, we decided that it doesn't worth to have Kagami Markdown-rendered documentation at all. Since it is a CLI utility app with no stable API (at least for now), it isn't worth effort to keep the documentation up-to-date with manual approach. As for automatic approach, #3510 shown that it doesn't work well: if we focus on Markdown-oriented help message, we have to sacrifice the usability of CLI-oriented help message. We decided that CLI interface has a higher priority.