fix: avoid superposition of bloch sphere in the tree

[12rambau]

Fix #14

Plotting analysis

I set the background of the tree figure to blue and the spheres in red to better understand what we were looking at:

What we see is:

• the sphere squares are overlapping on each other
• the top [0> is cut by the subplot

solution

for single display (separate calls to bloch_points) I left everything as it is. my logic being: there is no overlapping when there is just 1 sphere.

For tree I left the default background of the fig (transparent/white depending on the env) and set the background of the sphere axes to transparent white. By doing so we don't see the overlapping anymore.

before	after

Bonus

• moved the badges in the example section to look nicer in colab
• modified the image in the readme to use the one with transparent background

PS

@alice4space, I left the PR in draft state so that you get the PR -> review -> merge system. Once you will approve my PR I will turn it "ready for review" and you'll be able to merge it