Don't use numbers in tutorial titles

[jsignell]

From @jbednar:

Do you think it's feasible to make our user guides and tutorials link by name and not number? Right now we use the numbers to keep things in order, which helps .ipynb users go through things one by one in the right order. But the numbers are also used on the website, where they have the annoying feature of breaking links whenever we renumber. Could we strip the leading digits when we generate the website so that URLs never show them?
Switching to that would of course break all our existing links, but I think that would be worth it for the future.

Note to implementer. I think this would be a flag that could be enabled.