Refine rapidoc visual documentation

[juliasilge]

In #86 we set up some customized rapidoc with the hex sticker, etc:

We should come back and refine this at some point for better accessibility. Specifically:

• the endpoint description (like "Health check") does not have enough contrast
• could add alt text for the logo?
• could try to keep the containers from filling the whole page width:

.m-markdown p:not(:first-child) {
    margin-block-start: 24px;
    max-width: 80ch;
}