Differentiate tab icon for documentation

[lcawl]

The tab icon for Kibana was recently changed (elastic/kibana#64695) so that it looks almost identical to the documentation tab icon:

In this example, the first tab is Kibana. The second and fifth are documentation.

While we're reconsidering the design and location of our documentation, could we also consider making it stand out more when we have many tabs open in our browsers?

[gtback]

We'll need someone to decide on a different logo. We could also prefix all the page titles with "Docs", but I think that violates generally-accepted best practices for web design.