Rollup merge of #24239 - steveklabnik:editing_pass, r=steveklabnik

 Now that the new TOC has landed, I've started doing an editing pass to get the old content into the right shape. I felt this introduction was significant enough to send as its own PR, though, as it's the introduction.

It's possible that we may just want to replace 'the intro' with this directly, but this PR doesn't do that.