class_docstr: Custom directive to add an "allowmissing" flag to the include
  directive.
  This avoid errors if we want to include a file that might not exists.
code: def run(self):
      if "allowmissing" in self.options:
          env = self.state.document.settings.env
          rel_filename, filename = env.relfn2path(self.arguments[0])
          if os.path.exists(filename):
              return SphinxInclude.run(self)
          else:
              return []
      else:
          return SphinxInclude.run(self)
  
imports: import os
  from sphinx.directives.other import Include
  from docutils.parsers.rst import directives
label_desc: Use image node_id 1 for calling the Include obj's underlying member method code with example usage: obj.run() and returns: SphinxInclude, SphinxInclude, list