markdown docs: add section 'Available commands'

[deining]

When requesting help for a command with subcommands, a section Available commands is shown in the help output.
E.g. see message help for hugo's convert command:

hugo convert --help
Convert your content (e.g. front matter) to different formats.

See convert's subcommands toJSON, toTOML and toYAML for more information.

Usage:
  hugo convert [command]

Available Commands:
  toJSON      Convert front matter to JSON
  toTOML      Convert front matter to TOML
  toYAML      Convert front matter to YAML
...

When generating markdown docs for this command, this section Available commands is missing.
This PR addresses this issue by adding a section Available commands to the generated markdown output.

With this PR applied, the .md output for hugo's convert command now looks like:

## hugo convert

Convert your content to different formats

### Synopsis

Convert your content (e.g. front matter) to different formats.

See convert's subcommands toJSON, toTOML and toYAML for more information.

### Available commands

* [toJSON](/commands/hugo_convert_tojson/)	 - Convert front matter to JSON
* [toTOML](/commands/hugo_convert_totoml/)	 - Convert front matter to TOML
* [toYAML](/commands/hugo_convert_toyaml/)	 - Convert front matter to YAML
...