Add flexbox tutorial showing how to realise vertical placements of elements

[benjaminweb]

Adds examples going through different flex box alignments and other characteristics.

This clarifies greatly for example how to use grow and space for vertical placement of elements.

(I realised this by working through https://css-tricks.com/snippets/css/a-guide-to-flexbox/)

Users benefit from this by copying and starting from a working example instead of fiddling around to find the (yet undocumented) solution for the required layout.

This is of course a working draft. Let me know what could be added.
Also, this could be used as template for defining vertical placement functions in atomic-css directly like center.

[benjaminweb]

/layout is now:

[benjaminweb]

/text is now:

/whitespace-text is now: