class_docstr: Sort example gallery by title of subsection.
  Assumes README.txt exists for all subsections and uses the subsection with
  dashes, '---', as the adornment.
code: def __init__(self, src_dir):
      self.src_dir = src_dir
      self.regex = re.compile(r"^([\w ]+)\n-", re.MULTILINE)
  
file_path: scikit-optimize/doc/conf.py
imports: import warnings
  import os
  import re
  from packaging.version import parse
  import sys
  import skopt
  from github_link import make_linkcode_resolve
  import sphinx_gallery
label_desc: Use image node_id 1 to create a new SubSectionTitleOrder object  with example: obj = SubSectionTitleOrder(src_dir)