Add tutorial 11 to the index

[funkyrailroad]

Add tutorial 11 to the index. It was missing and not accessible from the tutorial menu.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks for the contribution; tutorial-11 not being in the index is intentional, as it's a placeholder for a future tutorial step that we've been meaning to write for a while.

Realistically, what we should probably do is:

1. Delete the "tutorial-11" stub
2. Rename the tutorial steps from 7 onward so that they're "topic" sub tutorials - essentially make "tutorial 7" like tutorial 5, in that there are multiple "paths" you can take. Steps 1-6 need to be done in a specific order; but steps 7-10 are pretty much standalone, and don't need to be done in order - you could do step 10 because you're interested in appearance customisation, but not do step 9 because you don't have a testing requirement.