Optional slug to unbind the notebook from the generated url name

[TuanaCelik]

In this PR we introduce an optional "slug" entry in the config.
This has some advantages:

• The url structure is no longer bound to the name of the notebook
  But also disadvantages:
• The generated markdowns would no longer necessarily share names with the notebook. So It's not so easy to find them if you're looking for them. You would have to go and look at the index.toml to figure out which .md file corresponds to it

[masci]

I think for new tutorials the problem of having a slug sensibly different from the notebook filename won't be that much of a problem, I'm 👍

[TuanaCelik]

The PR now includes an example "slug" for tutorial 1 which sets the slug to the one it has always been in HSH.
And an additional "Slugs" section in Contributing.md