Customise Read The Docs to match SSI branding (e.g. colours, logo)

[shoaibsufi]

Describe the change being requested
Customise the Read the Docs (RTD) theme to match SSI branding (e.g. colours, logo)

State the reason for the change
The branding will help with credit and recognition

Describe the impact of the change on other parts of the SSI-EOG
This is mainly focused on customisations to the RTD theme

Describe any alternatives you have considered (if applicable)
Just add the logo (we may still do this if RTD theme changes are too complex to keep in sync with the main RTD Sphinx theme)

Additional context
Suggest from Fellow Robin Long on the first release of EOG on 2020-12-14.

[shoaibsufi]

According to https://sphinx-rtd-theme.readthedocs.io/en/latest/configuring.html a logo can be added

Customisation of CSS is also possible but we might need help from the SSI RSG to do this.

Using configuration will allow us to be able to keep benefitting from upstream development of the RTD theme rather than forking it - so using configuration changes is preferred.