Release 3.x version (update repo docs, wiki, send out news to Code Alliance etc.)

[buep]

This issues represents work that is not automated in the release process:

• Make sure to highlight plugin is not compatible Make the plugin warn about not being backwards compatible in the update center #95
• Update plugin wiki page:
  • Release note section on version 3.x
  • migration guide, either link to md file in repo or wiki page section
  • clean the sections not relevant any more related to the build wrapper, as we're now a scm extension
  • clean out things related to multi scm
  • clean out examples or renew them, from the console as the plugin writes something else
  • explian failed, nothing to do
• Document the issues with the Jenkins file and merges if using pipeline jobs, see explanation here that can be copied and pasted more or less Handle Jenkinsfile needs merge and/or Jenkinsfile have changed on integration branch #59 (comment)
• remember to explain how to migrate UI created jobs to the new setup
• make sure to document all three kinds of scripted usage (Declarative pipeline, scripted pipeline and job dsl)

[buep]

Here's a draft @MadsNielsen https://docs.google.com/document/d/1bS-G293fmsL53uo3GA2MRado-hWlWGgUUOZG7id18IQ/edit# that will replace the wiki page.

We will need to add more section based on the above checklist

[buep]

Will you contribute ?

[buep]

Hi @MadsNielsen I updated the wiki... will you review it?

Especially try the scripted job examples so we know they are good, when you anyway do some manual validation.

We are still missing:

• updating the sections with orange color headers - fix, edit or remove and make them black header text
• I'm stilling missing the two pictures in the migration guide... could create them on my phone connection today.
• We should add a default git scm configuration image under the configuration section showing simple default setup, that matches the described git work flow.

[buep]

And we need to describe how it works with credentilals and that it is smart is uses the same credentilas from the git scm setup, because this is usually what you want.

[MadsNielsen]

I've added the following

1. A small section on credentials
2. Added two pictures to the migration guide
3. Added default configuration images to the recommended work flow.

Only thing now mising is the actual release.