GT: Update doc toggle icons

[adonahue]

This is a nice-to-have, not a requirement.

Update the doc icons in GT with these:

[mlippert]

For material design icons see https://materialdesignicons.com/ and filter on "file" (or look over the entire set)

I think perhaps file-remove or file-remove-outline to hide and file-plus or file-plus-outline or file-document-outline to show matches what @adonahue is suggesting w/ the icon images in the description?