chore: change netlify script to use Makefile

[johncowen]

Back in https://github.com/kumahq/kuma-gui/pull/2691 we moved all our dev scripts to a Makefile and noted:

Once we've merged this we can update this 'hook' in the Netlify web GUI to use the make file instead, we can then PR one last PR to remove this last package script.

I kinda forgot that we are using the netify.toml file instead of the web GUI, so I could have probably done this in the above PR but 🤷

As long as the preview site is being deployed here, then it should be fine (seems like it is to me)

I also updated related docs in CONTRIBUTING.md

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