Fix broken documentation URLs in CLI commands (#14651)

 ## Problem

Several CLI commands contain documentation URLs that return
404 errors because they reference non-existent README.md files
in chart directories. This creates a poor user experience when
developers try to access configuration documentation through CLI
help commands.

The affected commands are:
- linkerd upgrade: references charts/linkerd2/README.md
- linkerd multicluster install: references linkerd-multicluster/README.md  
- linkerd multicluster link: references linkerd-multicluster-link/README.md
- Chart documentation template also references broken README links

 ## Solution

Updated all affected CLI help text and chart documentation to point 
directly to the corresponding values.yaml files, which contain the 
actual configurable values that users need.

Changes made:
- Fixed URL in cli/cmd/upgrade.go from charts/linkerd2/README.md to 
  charts/linkerd-control-plane/values.yaml
- Fixed URL in multicluster/cmd/install.go from linkerd-multicluster/README.md 
  to linkerd-multicluster/values.yaml
- Fixed URL in multicluster/cmd/link.go from linkerd-multicluster-link/README.md 
  to linkerd-multicluster-link/values.yaml  
- Updated chart template in charts/linkerd-control-plane/README.md.gotmpl 
  to reference values.yaml files

Validation

Manually tested the following commands to verify help text shows correct URLs:
- linkerd upgrade --help
- linkerd multicluster install --help
- linkerd multicluster link --help

Verified that all updated URLs now point to existing files containing 
configuration documentation.

Fix: #14652 

Signed-off-by: Beza <[email protected]>

Author: bezarsnba

charts/linkerd-control-plane/README.md.gotmpl

@@ -120,9 +120,9 @@ extensions. Check the corresponding docs for each one of the following
 extensions:
 
 * Observability:
-  [Linkerd-viz](https://github.com/linkerd/linkerd2/blob/main/viz/charts/linkerd-viz/README.md)
+  [Linkerd-viz](https://github.com/linkerd/linkerd2/blob/main/viz/charts/linkerd-viz/values.yaml)
 * Multicluster:
-  [Linkerd-multicluster](https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster/README.md)
+  [Linkerd-multicluster](https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster/values.yaml)
 
 {{ template "chart.requirementsSection" . }}
 

cli/cmd/upgrade.go

@@ -77,7 +77,7 @@ plane. The default values displayed in the Flags section below only apply to the
 install command.
 
 The upgrade can be configured by using the --set, --values, --set-string and --set-file flags.
-A full list of configurable values can be found at https://www.github.com/linkerd/linkerd2/tree/main/charts/linkerd2/README.md
+A full list of configurable values can be found at https://www.github.com/linkerd/linkerd2/tree/main/charts/linkerd-control-plane/values.yaml
 `,
 
 		Example: `  # Upgrade CRDs first

multicluster/cmd/install.go

@@ -75,7 +75,7 @@ func newMulticlusterInstallCommand() *cobra.Command {
   linkerd multicluster install | kubectl apply -f -
 
 The installation can be configured by using the --set, --values, --set-string and --set-file flags.
-A full list of configurable values can be found at https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster/README.md
+A full list of configurable values can be found at https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster/values.yaml
   `,
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			if !ignoreCluster {

multicluster/cmd/link.go

@@ -88,7 +88,7 @@ each cluster and applied to the other.`,
   linkerd --context=east multicluster link --cluster-name east | kubectl --context=west apply -f -
 
 The command can be configured by using the --set, --values, --set-string and --set-file flags.
-A full list of configurable values can be found at https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster-link/README.md
+A full list of configurable values can be found at https://github.com/linkerd/linkerd2/blob/main/multicluster/charts/linkerd-multicluster-link/values.yaml
   `,
 		RunE: func(cmd *cobra.Command, args []string) error {