Use urfav Command struct directly to generate docsgen-cli

[masih]

The docsgen-cli builds binaries first, then executes them to get the usage, recursively for all sub commands. The process is less than optimal:

• logic to parse the usage is error prone and makes implicit assumptions (e.g. #12705)
• The make docsgen-cli target is slower than we want (even though it was made about 5X faster by porting over to go), and the majority of time is spent on building binaries.

We already have Go structs for commands. We should be using those (along with urfav's built-in helper generators) to generate the docs. This way the docs generation would be far faster and significantly less error prone.

To do this however, we should refactor the cli package to make some commands exported so that they are accessible from the docsgen-cli script.