docs: update dev docs

[smeijer]

I've collected/generated some information over the time, which we should add somewhere to make contributions easier:

• when needing components like dialogs, menu's, tabs, tooltips, etc, we should use @reach-ui when a library is required.

• for icons, we use octicons

• for illustrations, we use svg's from undraw, with the color #EDF2F7 and opacity of .5

• component hierarchy:

  

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I'll try to keep this issue updated until we have a better place. If you notice something missing, please comment below.