Add table of contents for tutorial

[rschristian]

User brought up in slack that the lack of a table of contents makes it difficult to navigate, which I think is valid: https://preact.slack.com/archives/C3NMBSJKH/p1661337174825169?thread_ts=1661328160.791249&cid=C3NMBSJKH

Could probably add one on the page with the intro blurb: https://preactjs.com/tutorial/

[developit]

That's a good idea - I couldn't figure out how to add one on the tutorial pages themselves because there's not much real-estate left there, but on the index page would be nice.