Check if `sphinx-book-theme` can be updated

[micokoch]

It seems that sphinx-book-theme is not updated when it builds the webpage, because when I run the page locally, the header configuration is slightly different (e.g. there is a light/dark mode button and the search button is next to the other buttons).

In other words, my local version seems to be running the more recent version of sphinx-book-theme which looks more like what is here.

[micokoch]

This is important because it seems they will soon update the "suggest edit" button in a newer release (which is currently problematic).

[micokoch]

Finally able to fix this by updating requirements.txt, using pip freeze > requirements.txt