AG - Improve Admin guide: "1. Customizing the devfile and plug-in registries" section

=====1.1.1. Building a custom devfile registry

1)
> These are available on the CodeReady Workspaces user dashboard when selecting Create Workspace
`Create Workspace` is not present any more. The section is now called `Get started`

2)
> 3. Add valid content in the devfile.yaml file. For a detailed description of the devfile format, see
LINK IS INCORRECT 404

3)
> 5. Build the containers for the custom devfile registry:
It doesn't say from which directory it should be done (The answer: codeready-workspaces/dependencies/che-devfile-registry/)

4)
> docker build -t my-devfile-registry .
Wrong command. It fails.
Command which worked:
docker build -t my-devfile-registry -f build/dockerfiles/Dockerfile .
(Please notу item (4) which defines the root directory to build from)

5)
Should we use rhel.Dockerfile for the registry or up to users?

6)
In the docs we can notify user that they clones master devfile registry with latest list plugins. Maybe they want's to have list plugins for specific version.

===== 1.1.2. Building a custom plug-in registry

7)
> See the “Using a Visual Studio Code extension in CodeReady Workspaces” section or the README.md file in the eclipse/che-plugin-registry
Should be a link

8)
> README.md file
Should be a link

10)
> Build the containers for the custom plug-in registry:
From what directory? (should be codeready-workspaces/dependencies/che-plugin-registry)

===== 1.1.3. Deploying the registries

11)
> This is true on Minikube, for example, if before running the docker build commands, the user executed the eval ${minikube docker-env} command (or, the eval ${minishift docker-env} command for Minishift).
Remove this paragraph from CRW doc. It only make sense for upstream doc.

12)
> Deploying registries in OpenShift
> To deploy the plug-in registry using the OpenShift template, run the following command:
The command is not correct. Right one (Keep env variables definitions above):
oc new-app -f openshift/che-plugin-registry.yml \ -n "${NAMESPACE}" \ -p IMAGE="${IMAGE_NAME}" \ -p IMAGE_TAG="${IMAGE_TAG}" \ -p PULL_POLICY="IfNotPresent"

13)
> The OperatorHub installation method deploys CodeReady Workspaces to the users current project.
Wrong. It deploys to `openshift-workspaces` project

14)
> 2. The devfile registry has an OpenShift template in the `deploy/openshift/`
Where is the folder?
> oc new-app -f openshift/che-devfile-registry.yml
There is no `openshift/che-devfile-registry.yml` file.
Location of che-devfile-registry.yaml for upstream: https://github.com/eclipse/che-devfile-registry/blob/master/deploy/openshift/che-devfile-registry.yaml
THERE IS NO WAY TO BUILD DEVFILE REGISTRY FOR CRW because the file is missing in the repo for CRW. We cannot continue testing in full scope with this scenario.

15)
> The devfile registry has an OpenShift template in the deploy/openshift/ directory of the GitHub repository. To deploy it, run the command:
The command is wrong. Right one (Keep env variables definitions above):
oc new-app -f openshift/che-devfile-registry.yml \ -n "${NAMESPACE}" \ -p IMAGE="${IMAGE_NAME}" \ -p IMAGE_TAG="${IMAGE_TAG}" \ -p PULL_POLICY="IfNotPresent"

16)
> $ URL=$(oc get -o 'custom-columns=URL:.spec.rules[0].host' \
-l app=che-plugin-registry route --no-headers)
Command failed (returned empty string)

17)
> or the devfile registry):
> $ oc get \
The command is not correct and alignment seems to be broker (even different fonts is used!)
Correct one:
oc get -o "custom-columns=URL:.data['CHE_WORKSPACE_PLUGIN__REGISTRY__URL']" --no-headers cm/che -n openshift-workspaces

18)
> c. If they do not match, update the ConfigMap and restart the CodeReady Workspaces server.
We should update CR, not configmap

19)
> oc scale --replicas=0 deployment/che
Should be codeready

===== 1.2. Including the plug-in binaries in the registry image

1. Che plugin and devfile registry doesn't work on the user dashboard they were broken from previous steps... Ok have list env variables and we don't have information where is the place to include them. We don't have information where is current folder dir to launch sed commands. Command looks wrong.

2 We don't have information how to rebuild plugin registry and docs is unclear at all. We should have information how to build offline registry.

"Building and running a custom registry image" link should be more specific the link to the plugin registry sebscection. For now it points to whole docs with both plugin and devfile registry.

1.3 We should have on the github example with plugin. We should not say to user, find any example and modify some fields. It's looks not friendly.

1.3.6 We have long command to rebuild registry but it looks too hard. We need specify each step to be more clear.