Create link to documentation.

def link_type(arg_type, arg_name=None, include_bt:bool=True):
    "Create link to documentation."
    arg_name = arg_name or fn_name(arg_type)
    if include_bt: arg_name = code_esc(arg_name)
    if belongs_to_module(arg_type, 'torch') and ('Tensor' not in arg_name): return f'[{arg_name}]({get_pytorch_link(arg_type)})'
    if is_fastai_class(arg_type): return f'[{arg_name}]({get_fn_link(arg_type)})'
    return arg_name