More webpage improvements

[avivajpeyi]

• [ ] Use jupyterbook-css on the individual TOI pages (fixes #165 )
• [ ] Disable pointless left sidebar scroll (see https://github.com/executablebooks/sphinx-book-theme/issues/732)
• [ ] open-in-colab button (#233 )
• [ ] Add link-to-paper (https://sphinx-book-theme.readthedocs.io/en/stable/sections/header.html#an-eample)
• [ ] Disable 'dark-mode' option (https://github.com/executablebooks/sphinx-book-theme/issues/750)

Some ✨ extra ✨ stuff that would be nice

• [ ] Add utterances on the individual TOI pages (https://sphinx-book-theme.readthedocs.io/en/stable/reference/comments.html#utterances)