Add docs generation to benthos cli as a command

[lucasoares]

A few months/years ago I made this PR: #1566

I think it got lost because there was no issue linked to it.

The use case here is pretty simple: to be able to generate documentation for plugins created outside the benthos repository.

My team currently has its own benthos distribution and we have a lot of custom components. Many of them are custom components only useful for our team. We want a documentation containing these components.

We will only bring to benthos as contributions the components we think may help other people, but many of them are only useful for our company.

Currently, all the code used to generate the markdown documentation is already bundled with the Benthos binary (on /internal/docs which is imported by many other packages), and having the docs cli command would be extremely helpful for me.

I noticed a few things got changed by @Jeffail on the docs generation since my PR creation and I fixed it. The PR is kind of finished, but needs approval so we can delete unused code from benthos_docs_gen.