Expand tutorial to include segmentation of original dataset and extraction of traits

[Abe404]

As discussed in #60 (reply in thread), the colab tutorial could be extended with a similar step-by-step guide including checking performance using the metrics plot, segmenting the original images, checking the composites and extracting traits as CSV (or preparing images for RVE).

I actually don't think this needs to be a colab tutorial and could be a simple step by step guide available as HTML or PDF.

It could also be useful to explain that waiting for 60 epochs without progress doesn't matter than much as many users on free colab are getting kicked off (#67 (reply in thread)) before this happens.

[Abe404]

I have created a short mini-guide to help users with the overall process:
https://github.com/Abe404/root_painter/blob/master/docs/mini_guide.md

I will leave this issue open as I am aware that even more documentation (going into more detail about each step, including screenshots etc) and videos would be beneficial. I just don't have time right now (but hopefully can do something more comprehensive in a few months).