Better documentation of IPCC colors

[danielhuppmann]

The pyam plotting module has a PYAM_COLORS attribute that holds has the colors used in several IPCC reports (see #650 for AR6 WG3). There is also a tutorial on how to use the colors for plotting.

It would be great to have better documentation of the colors as part of the docs and think about how to make the tutorial more informative.

cc @jkikstra @gidden @byersiiasa

[byersiiasa]

thank you @danielhuppmann, will add it to my list. - will take a look. Perhaps @AlKhourdajie wants to check also.

[AlKhourdajie]

Thanks both, shall do, on the list too.