Contributor guide and extensions: remove content for unavailable features on instances with devworkspace, in the master branch

[l0rd]

Is your enhancement related to a problem? Please describe

We struggle to maintain contributor guide and extensions documentation. Those guides are not updated and considered that our involvement in the development of the IDEs themselves has switch to support vanilla IDEs (VS Code and IntelliJ) our capacity to maitain those Theia related doc is even less.

Describe the solution you'd like

Remove content from these guides in the master branch and eventually move some specific articles somwhere else

Here is the current list of the contributor guide and extensions articles. In bold the ones to move somewhere else. The rest need to be removed definitely.

Contributor Guide

• branding che-theia (move to admin guide)
• developing che-theia plugins
• testing che-theia plugins
• adding support for a new language
• adding support for a new debugger
• che extensibility reference

Extensions

• Eclipse Che4z
• OpenShift Connector (propose to openshift connector team to own this doc and move it to their doc)
• Telemetry (move to admin guide)
• Java Lombok

Describe alternatives you've considered

No response

Additional context

No response