Fix all the TODO's

This issue will probably never close; it's a perennial reminder that the TODOs in the documentation need addressing.

Help doing this is very welcome; you can **just fix them one at a time if you like**!

Some things are simple "check this example works" - so if you can check it and it works you can delete the text and submit that as a PR 👍

Others are adding examples - which you'd also be very welcome to submit as a PR 👍

Basically what I'm trying to say is:

# PRs welcome!

Start by locating the TODOs in the documentation and choose one item to investigate. Verify whether each example works, remove resolved TODO text, or add the requested example; done means the selected documentation TODO is addressed and the relevant example has been checked.

Written by the indexing model from the issue text.