Hi !
I'm guessing by documentation panel you mean : ctrl/cmd+space -> "Docs: Search language docs" ?
Don't think there is any support for this out of the box.
Feel free to submit pull requests to the
https://github.com/LightTable/HTML and
https://github.com/LightTable/CSS plugins if you have an idea on how/where to actually find docs and keep them up to date.
I don't think anyone in the core team has bandwith to give this priority in the near future, but we'll be happy to help anyone willing to give it a go !
cheers
Magnus