Fix broken code examples and markdown in README
- Add missing closing code fence after getPreviousSiblings(), which left
the rest of the section rendering as one big code block
- insertAdjacentHTML: remove stray space in position arg ('beforebegin ')
- Fix TOC anchor [6.5] pointing to #6.4 instead of #6.5
- Promise.all().then(): wrap the destructured arrow-fn parameter in
parentheses so it is valid syntax
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>