HTML validation

1. Check that validation of the theme docs home page displays:

   Document checking completed. No errors or warnings to show.

   (It currently displays 81 Infos. Some of them come from jekyll-seo-tag, but most are from the HTML that the home page uses to display the avatar images of contributors.)

2. Check that the theme docs have no broken links, except for examples such as https://just-the-docs.github.io/just-the-docs/docs/index-test/another-page.