[docs] Parallelization tutorial

[da-woods]

I've written a parallelization tutorial. It doesn't cover that much new ground from the main parallelization documents, but hopefully it present a separate example of each use case, and is a little less of "here are the parameters, good luck" than the main parallelization reference

Kind of closes #5125

[matusvalo]

Huge +1 for this tutorial! I found just some small typos for now. One note regarding the PR, pure python is missing but this can be done even in next PR + the tutorial uses also new keyword which is missing in pure python mode.

[da-woods]

pure python is missing

Mainly laziness...

the tutorial uses also new keyword which is missing in pure python mode

We could use a malloced array in pure Python mode (with a note that the code isn't quite identical). It doesn't really matter for the example whether the storage is a vector or an raw memory block.

[matusvalo]

Mainly laziness...

No worries. We can go even with this version and I am willing to migrate it to pure python mode afterwards.

[da-woods]

I've added the pure Python examples. Unlike the rest of the documentation I've put the Cython examples first. Mainly because I prefer it that way. Obviously with it being open source other people can come in later and swap that around but I thought I'd do it my way initially.

[matusvalo]

Another batch of possible improvements

[matusvalo]

LGTM

[da-woods]

I'm going to merge this. It's docs so it's not going to break anything...