We should generate doc comments for each variant of TerminalKind and NonterminalKind. More specifically, it should contain something like:

// This kind represents a `{ variant name }` node, with the following structure:
// {variant docs}

The initial ground work for this has been done in:

1. codegen_ebnf::EbnfModel type, where it can generate EBNF docs for each variant separately.
2. terminal_kind.rs.jinja2 and nonterminal_kind.rs.jinja2 where they have a commented out snippet like below. Maybe we can change variant to be a structure containing .name and .documentation instead:

        {%- for variant in model.kinds.terminal_kinds -%}
            {# variant.documentation | indent(prefix = "/// ", first = true, blank = true) #}
            {{ variant.name }},
        {%- endfor -%}