text: Title: singlehtml builder not available
  Body: ### Link to the documentation you'd like to improve
  
  https://jupyterbook.org/basics/build.html#types-of-build-outputs
  
  ### What to improve
  
  The information provided about the list of builders seems inaccurate: singlehtml builder doesn't seem to be available.
  
  ```python
  > jupyter-book --version
  Jupyter Book      : 0.11.2
  External ToC      : 0.2.3
  MyST-Parser       : 0.13.7
  MyST-NB           : 0.12.3
  Sphinx Book Theme : 0.1.1
  Jupyter-Cache     : 0.4.3
  NbClient          : 0.5.4
  > jupyter-book build --builder singlehtml .
  Usage: jupyter-book build [OPTIONS] PATH_SOURCE
  Try 'jupyter-book build -h' for help.
  
  Error: Invalid value for '--builder': invalid choice: singlehtml. (choose from html, dirhtml, pdfhtml, latex, pdflatex, linkcheck, custom)
  ```