Customise the RTD site to match iGEM branding

[noahsprent]

The RTD site currently uses the default RTD theme for Sphinx. We can and should customise it to make it look nicer and fit the iGEM branding: https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html.

[noahsprent]

https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html