Customise error pages

[PeterJCLaw]

Currently if you hit a 404 in the docs (or any other 4xx or 5xx error), such as https://studentrobotics.org/docs/missing, you're shown a page which exposes that we host the docs on github pages. We should provide custom error pages to maintain our branding.

[PeterJCLaw]

Either of the following seem like reasonable solutions:

• https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_intercept_errors (in the main website nginx config)
• https://help.github.com/articles/creating-a-custom-404-page-for-your-github-pages-site/ (in this repo)