Add `wp handbook gen-custom` for custom commands

[BrianHenryIE]

I want to use the same documentation style in my own plugins. I.e. wp handbook gen-custom my-command should generate markdown docs for my command, wherever I want them in my project.

NAME

  wp handbook gen-custom

DESCRIPTION

  Generate the documentation page for a custom command using the same templates as the official WP-CLI commands.

SYNOPSIS

  wp handbook gen-custom <command> [--output_dir=<output_dir>] [--verbose]

OPTIONS

  <command>
    The custom WP CLI command to generate the documentation for.

  [--output_dir=<output_dir>]
    Directory in which to save the documentation.
    ---
    default: ./docs/wp-cli
    ---

  [--verbose]
    If set will list command pages as they are generated.

• ./docs/ was chosen based on php-pds/skeleton
• gen-custom should maybe be gen-custom-command
• Documentation above needs examples – what WordPress maintained plugin has CLI commands?
• I need to consider how to handle subcommands
• Documentation for gen-all (which runs on @when before_wp_load) needs to make clear that it means all WP CLI native commands
• Tests

[ernilambar]

I think this would be a good candidate for separate WP-CLI package. Few months back, I needed similar for the commands in a plugin. I ended up copying and modifying the parser from this repo and used it to generate docs.