[docs] `index_chunks` undocumented in "Multithreading" page despite the promise to do so

[giordano]

The "Multithreading" section of the manual starts with

ChunkSplitters.jl/docs/src/multithreading.md

Line 3 in 7db0af6

The iterators `chunks` and `index_chunks` can be very useful in combination with `@spawn` and `@threads` for task-based multithreading. Let's see how we can use them together.

but then in the rest of the page there's no mention to index_chunks. Of course the function is documented elsewhere, but reading that page feels weird, since a promise to discuss index_chunks isn't maintained.