Sign created artifacts, document updated processes

[niloc132]

Deployed manually for testing at https://plugins.gwtproject.org/eclipse/gwt-eclipse-plugin/nightly/ - note that in the future this repo will likely not be signed, and only releases will be signed in this way.

The signing cert here is the same cert as is used by plugins.gwtproject.org itself - we can probably improve the experience here somewhat, but it does seem to be a step up from no signing at all.

Fixes #446

[niloc132]

Confirmed that PRs won't sign (without branch name check) https://github.com/gwt-plugins/gwt-eclipse-plugin/actions/runs/4214214640/jobs/7314529326

Confirmed that branches (without branch name check) will sign https://github.com/gwt-plugins/gwt-eclipse-plugin/actions/runs/4214248488/jobs/7314591205

Confirmed PR check works with branch name check https://github.com/gwt-plugins/gwt-eclipse-plugin/actions/runs/4214214640/jobs/7314529326

Confirmed that non-matching branches work with branch name check https://github.com/gwt-plugins/gwt-eclipse-plugin/actions/runs/4214331119/jobs/7314726087

Going to verify artifacts tomorrow, then this should be ready to land, subject to reviewer approval.

[niloc132]

Looks like this is working, please try downloading the repo zip at https://github.com/gwt-plugins/gwt-eclipse-plugin/actions/runs/4217845131 to take a look at how eclipse handles the self-signed cert I have configured for this project. Presently intending to only use that for non-release builds, but we could also use them for releases too. For releases, I had intended to use the same cert that is used for hosting plugins.gwtproject.org, until we have enough demand to buy a CA-signed code signing cert.

[protoism]

I tried, it's working! Here a snapshot