Document usage of `gum` crate (#7294)

* Document usage of gum crate

* Small fix

* Add some more basic info

* Update node/gum/src/lib.rs

Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>

* Update target docs

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Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>