Activity `<note>`s/sections should be automatically indexed

[40thieves]

Which module(s) and week(s) does this change affect?
Module(s): N/A
Week(s): N/A

What is the work that needs to be done?

Activity <note>s should have an index added to them automatically, incrementing from the first activity upwards.

I would probably prefer with A -> Z over 1 -> X to distinguish between activities and sprints - in other words I think it's easier to say "Sprint 1, Activity C". But then you potentially have to deal with what happens if you have more than 26 exercises.

Why is this work important to do?

All of the exercises in the React module are named Exercise A, Exercise B etc as I found it useful to explain to trainees that we're working on Exercise X. In a flipped classroom context it's also useful for trainees to say "I'm stuck on Exercise Y".

Maintaining these indexes is a pain and error-prone, so it would be useful to generate them automatically.

Who might need to know about this change?

@CodeYourFuture/global-syllabus