docs: Unnatural document appearance

[munechika-koyo]

The updated document appears to be weird (unmatched font, missing icons, code highlighting, etc.)
This seems to have occurred due to the incompatible customized theme.css with the latest sphinx_rtd_theme.

Here is the latest preview of the document with sphinx_rtd_theme v3:

Previous preview with sphinx_rtd_theme v0.5.2:

[CnlPepper]

I'm seeing everything render ok at www.raysect.org?

[CnlPepper]

That said... our style mods are, as you said, quite likely to be incompatible now. We can either pin the rtd package version to the old one or update the style sheet (if it is still needed?).

[munechika-koyo]

Thank you for your comment @CnlPepper
I improved the theme.css, which describes what is only required to customize, without interfering with the style that the RTD offers. (#457)
So, I think we should switch to the new method.