docstr: add link idx as anchor
  
  Args:
      text (str): text string written to pdf, with current
                  global indent level
      link (str): link idx created earlier for internal anchor
      on_click (bool): indicate if the list item (menu) click-able,
                          default False
  Returns: HTML String
code: def add_anchor(text: str, link=None, on_click=False):
      """
      add link idx as anchor
  
      Args:
          text (str): text string written to pdf, with current
                      global indent level
          link (str): link idx created earlier for internal anchor
          on_click (bool): indicate if the list item (menu) click-able,
                              default False
      Returns: HTML String
      """
      doc = Doc()
      with doc.tag("a"):
          if link is None and not (text is None):
              link = "%s" % text.lower().replace(" ", "-")
          doc.attr(href="#a%s" % link)
          if on_click:
              doc.attr(onclick="Section(event, 't%s')" % link)
          doc.attr(klass="tab_links")
          doc.text(text)
      return doc.getvalue()
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import datetime
  from yattag import Doc, indent
label_desc: Use image node_id 16 for calling the CustomHtml obj's underlying member method code with example usage: obj.add_anchor(text, link, on_click) and returns: doc