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

**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 ``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