Add Impala to config defined docs menu

[samredai]

The recent iceberg-theme that was merged uses a Hugo config based menu. This adds the external link to the Impala docs. Since logos are no longer being used, this also removes the impala logo image from the repo.

cc: @pvary

[pvary]

@samredai: Shall we add a Hive menu item as well?

Also this seems off:

{ name = "Presto", url = "../../how-to-release" , weight = 500 },

[samredai]

@pvary yes and yes, thanks for catching that! Opening a PR now 👍

[samredai]

@pvary with respect to adding Hive, that actually has its own internal page so when hive.md is copied over from the iceberg repo, it will automatically show up as an item in the leftnav. (Set by the menu: main in the front matter)

[pvary]

That's cool.

Maybe a link in the main menu would be good, somewhere between Presto, Trinto and Impala.