templates/ and parts/ replace the template hierarchy

5.9 on 25 January makes a theme a directory of HTML files, and the PHP template hierarchy stops being how a page is chosen.

theme/
  style.css          still required, still the header
  theme.json         the settings and styles
  templates/
    index.html       the only REQUIRED file
    single.html
    archive.html
    page-about.html
  parts/
    header.html
    footer.html

# no index.php. no functions.php, unless you want one.

The naming still follows the template hierarchy — single-product.html beats single.html beats index.html — so the resolution rules are the ones everybody already knows with a different extension. What is gone is the ability to put logic in a template, which is the actual change and is the subject of the next several notes.