Enhance the Push Updates (with Node.js) doc page and add it to the documentation menu

[marc-farre]

I've successfully tested https://docs.humhub.org/docs/admin/push-updates/
I now use it in production for a HumHub instance. Node is executed as a service with systemd.

I think it's an important feature for HumHub and that we should promote it.
See humhub/mail#34 and humhub/mail#213

I can send a PR for that.

[luke-]

Thanks for the feedback. Currently is rarely used indeed.

We should implement some more testing and support in our Saas - before pushing it in the docs.

[luke-]

• Its planned to use the "live" update system more extensively in future. (e.g. typing notifications). After this implementation it would be a good time to promote it.