Update docs that mention plugin-* and stacks-* to use udi image

Because of CRW-2464 and CRW-2463 (and CRW-2729), the plugin-* and stacks-* images have been collapsed into a single udi-rhel8 image.

Therefore docs need to be simplified to point users at this single image for starting workspaces.

Affected docs:

• Admin Guide 11.1. Defining the list of images to pull
• Admin Guide 11.3. Installing Image Puller using the CodeReady Workspaces Operator
• Admin Guide 11.5. Installing Image Puller on OpenShift using OpenShift templates

• EUG 4.1.5.4.10. Component type: dockerimage (id is correct, but wrong image shown - should not have Che in the CRW docs)
• EUG 4.1.5.4.14. Specifying container memory limit for components (id is correct, but wrong image shown - should not have Che in the CRW docs)
• EUG 4.1.5.4.15. Specifying container memory request for components (id is correct, but wrong image shown - should not have Che in the CRW docs)
• EUG 4.1.5.4.16. Specifying container CPU limit for components (id is correct, but wrong image shown - should not have Che in the CRW docs)
• EUG 4.1.5.4.17. Specifying container CPU request for components (id is correct, but wrong image shown - should not have Che in the CRW docs)

• EUG 5.1.3. Che-Theia plug-in metadata (id is correct, but wrong image shown - should not have Che in the CRW docs - quay.io/eclipse/che-machine-exec:7.9.2 ==> registry.redhat.io/codeready-workspaces/machineexec-rhel8:3.0)
• EUG 5.1.5.2. Remote plug-ins (id is correct, but wrong image shown - should not have Che in the CRW docs - eclipse/che-theia-dev:nightly ==> registry.redhat.io/devspaces/udi-rhel8:3.0)
• EUG 5.5.1. Testing a VS Code extension using GitHub gist (id is correct, but wrong image shown - should not have Che in the CRW docs - quay.io/eclipse/che-sidecar-* ==> registry.redhat.io/devspaces/udi-rhel8:3.0)
• EUG 5.9.2. Adding a language support plug-in to a CodeReady Workspaces workspace (id is correct, but wrong image shown - should not have Che in the CRW docs - quay.io/eclipse/che-sidecar-* ==> registry.redhat.io/devspaces/udi-rhel8:3.0)

• EUG 6.3.1. Configuring Python to use a non-standard registry
• EUG 6.3.2. Using self-signed certificates in Python projects
• EUG 6.4.1. Configuring Go to use a non-standard-registry (wrong image shown - should not have Che in the CRW docs)
• EUG 6.4.2. Using self-signed certificates in Go projects (stacks image is now udi image)
• EUG 6.5.2. Using self-signed certificates in NuGet projects (wrong image shown - should not have Che in the CRW docs)
• EUG 6.6. Using npm artifact repositories (wrong image shown - should not have Che in the CRW docs)

• Install Guide 3.4.2.1. Preparing an private registry
• Install Guide 5.3.3. Preparing an private registry