Smoothen automatable tasks

[knz]

• markup around diagrams to avoid markdown from going haywire
• js for copy-paste before examples
  • clean up extant examples before doing so
• div toc at the beginning
• automate the layout and indentation of SQL examples

[jseldess]

Automating the layout of SQL examples is now possible with sqlfmt. @mjibson has a PR open for this; I need to get back to it.

The first two items would be nice, but we should wait until after the Hugo migration to reassess.

[jseldess]

@knz, I've opened a distinct issue for SQL example formatting: #7059.

When you have time, please open distinct issues for the first two tasks as well. I'd do so, but I don't quite understand them.