Bring Che plugin-examples into Che 4

[ddementieva]

Once the che plug-in utilities on the CLI have been re-added into Che, we need to perform the following work:

1. Port the github.com/codenvy/che/examples to work on Che 4.
2. Move the /examples directory to be in assembly/src/assembly/examples/plugin
3. Update the assembly.xml file to have the /examples directory copied into root of Che assembly.
4. Move the /archetypes folder from Che 3 into new module labeled assembly-archetypes in Che 4 repo.
5. Update the archetypes to build properly
6. Change the Jenkins build system to automatically build and publish these archetypes.
7. Update Che 4.0 docs with how plug-ins now work in 4.

[ddementieva]

(Comment by Tyler Jewell)

[~vparfonov] I set a BV on this and would like to target it for 4.1. I can do some of it, but not the archetype part.