Improve documentation about volumeClaimTemplate workspaces
Improve documentation about volumeClaimTemplate workspaces

Add a short example and link to a full example of using volumeClaimTemplate
as a volume source in a workspace.

Requested in comment to PR https://github.com/tektoncd/pipeline/pull/2326#issuecomment-616762191
that
fixes #1986

tektoncd/pipeline,tektoncd/pipeline