Update garden dwave notebooks formatting

• Fix the link to companions.png ( the file in the repo is a png not an SVG ) Currently, they are not rendered in the jupyternotebook because of this:

• Split cells where it makes sense; for eg the code snippet

#a)
df = pd.read_csv('./companions.csv').set_index('Species')
df

is better to be isolated into its own cell as the output of the second line would not be seen otherwise.

so before:

after:

• simplify the make_histogram function example