Add icon to navigation tree

[benoit74]

Icon on the first item of the navigation tree is missing

In the ZIM

Online

DevDocs compiles these into a spritesheet that has a nondeterministic name so it's not easy to pull just the icon we want out, but it might be possible to grab the whole sheet. The icons are also available in the DevDocs repo: https://github.com/freeCodeCamp/devdocs/tree/main/public/icons/docs, but there's no license information for the icons which may differ from the content.