Improving the documentation layout

[haffi96]

The current documentation is a little difficult to navigate for a couple of reasons:

• The table of content is overflowing on the text (as highlighted in #1128)
• The doc is a long one pager...
• The search box is right at the bottom

Most of these problems would be solved by simply using a more updated sphinx theme (https://sphinx-rtd-theme.readthedocs.io/en/stable/index.html)

I've opened a PR for this. See #1135