Split up graphic design guidelines and add more docs on custom pipeline logos

[jfy133]

Because that page is getting pretty big, and we are getting more requests for pipeline logos.

I think there might be a purpose of splitting 'set' nf-core stuff (e.g. logo/fonts/colours) with more crerative things (pipeline logos, workflow diagrams), so we could maybe merge a couple of these pages again if you want.

@netlify /docs/guidelines/graphic_design/colours

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[jfy133]

Also note I made up 'on the fly' the custom pipeline diagram guidelines, of the feedback I've been given in the past.

Feel free to modify directly if you disagree.