Revamp Che 7 administration guide

[slemeur]

Goal

The goal of this issue is to revamp the administration guidelines of Che 7.
Outline will need to be rework and explanations needs to be adapted/reviewed/reworked

Subtask:

• [docs] Document All fields of the CheCluster CR in che-docs and CRD - [docs] Document All fields of the CheCluster CR in che-docs and CRD #13886
• [docs] Document the various places where Che can be configured (Che Custom resource definition; it could be in the Config Map; or in the che.env) - https://issues.redhat.com/browse/RHDEVDOCS-1581

[slemeur]

cc #13886

[nickboldt]

Closing in favour of https://issues.redhat.com/browse/RHDEVDOCS-1581