Fix thumbnails for notebooks on doc website

[ninamiolane]

What?

The tutorials webpage of the documentation website does not display the thumbnails properly. We should fix this.

https://pyt-team.github.io/toponetx/tutorials/index.html

Why?

The tutorials webpage is one of the first pages that users see. It should look clean.

Where?

The nbsphinx_thumbnails dictionary could be updated in:
https://github.com/pyt-team/TopoNetX/blob/main/docs/conf.py

Alternatively, the tag nbsphinx_thumbnails could be used directly in the jupyter notebook as meta data of a cell. Note that this would override the nbsphinx_thumbnails dictionary from conf.py

There is a difficulty emerging from the fact that the notebooks are loaded in docs/tutorials through the symlink notebooks.

How?

Some information can be found on the website of nbsphinx which is the package used to create such thumbnails:
https://nbsphinx.readthedocs.io/en/0.9.1/gallery/thumbnail-from-conf-py.html

[devendragovil]

I found a way to enable the thumbnails. The PR #179 fixes this. However, I am trying to come up with a better method. The Symlink is indeed presenting a lot of issues.

[devendragovil]

@mhajij @ninamiolane PR #179 has fixed this issue.