Mechanism to refer to app-icons in documentation

[jleibs]

It would be very helpful in docs to be able to refer to the app icons such as the "reset" or "add" icons. Ideally we would sync the icons from re_ui into landing somehow and put them at a location we could refer to.

[emilk]

While we're at it, let's store each icon as both .png and .svg side by side. That way we can use the .svg in the docs, and have a source to use if we need higher quality icons in the viewer when we finally work on this:

• https://github.com/rerun-io/rerun/issues/2960