Add html theme

[shenvitor]

As the title says.

[redeboer]

Before switching between themes too frequently, it's better to decide on one theme. I would use either

• https://github.com/pydata/pydata-sphinx-theme
• https://sphinx-book-theme.readthedocs.io

The latter has the advantages that it works best with myst-nb as it is developed by the same group. Note that sphinx-book-theme is just wraps pydata-sphinx-theme, so using a subset is fine as well.

[shenvitor]

Layout can be modified in the future. https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/layout.html#header-navigation-bar

Looks nice 😏 Approved 🔥

Just a thought though: it seems that pydata puts each page on the top navigation bar, which isn't really scalable (you can't scroll horizontally). I sure it's possible to move them to the sidebar, so we can look at that once there are more notebooks. (This is incidentally why I prefer sphinx-book-theme — it has some convenience settings that make it easier to move the pages between sidebar and navbar.)