Add pushing to update site to Github actions

[vogella]

I recently setup the push to an update site via simple Github actions, see here https://github.com/vogellacompany/eclipse4book/blob/master/.github/workflows/maven.yml

IIRC you are using a shell script outside Github for this. Why not push it via the Github action?

[de-jcup]

I was forced a long time to use Maven 1.0, write extensions in XML etc. (yes I know it is a very long time...) but this is something that influenced my life made me biased and I embraced gradle so much at all ...

Maven 3.0 is much better - I know ...

The gradle build I am using is only for building and testing the "non-eclipse" parts (I seperate "normal" logic and eclipse parts).
Reason: I tried to use maven tycho and setup parts etc. But this was very error prone and it just did not work. So I tried this approach. I always thought about creating something like tycho for gradle as well, but I am absolutely running out of time (private issues), so this will normally not happen in next years.

Building and Deploying is currently done complete manual - so I use a very old eclipse RCP as a base and build the plugin there the old school way (using the IDE editors for plugin.xml and site.xml...)

I just created an issue to migrate the build to maven and automate it:
#371