Remove documentation directory?

[ericonr]

The documentation directory was last updated in 2020, and seems to have been replaced by the docs one, but it's still in the ADCore tree. For users of the repository, they could accidentally refer to it and end up using outdated/wrong information, or think some documentation is missing; and for developers, they could accidentally document things in the wrong directory tree.

It seems like removing the tree entirely would help in avoiding issues. A quick grep didn't reveal any references to it, but it would probably be best to confirm.