Generate user guide command links

This PS adds docs.airshipit.org link to long description
of the airshipctl root command output. The implementation
is done in-line with kubectl command.
https://github.com/kubernetes/kubectl/blob/master/pkg/cmd/cmd.go#L479

Closes: #281
Change-Id: Id65c53a72a840221c47677bacf9651f9430da276

cmd/root.go

@@ -34,6 +34,11 @@ import (
 	"opendev.org/airship/airshipctl/pkg/log"
 )
 
+const longRoot = `A unified entrypoint to various airship components
+
+  Find more information at: https://docs.airshipit.org/airshipctl/
+`
+
 // RootOptions stores global flags values
 type RootOptions struct {
 	Debug             bool
@@ -54,6 +59,7 @@ func NewRootCommand(out io.Writer) (*cobra.Command, *RootOptions) {
 	rootCmd := &cobra.Command{
 		Use:           "airshipctl",
 		Short:         "A unified entrypoint to various airship components",
+		Long:          longRoot,
 		SilenceErrors: true,
 		SilenceUsage:  true,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {

cmd/testdata/TestRootGoldenOutput/rootCmd-with-default-subcommands.golden

@@ -1,5 +1,7 @@
 A unified entrypoint to various airship components
 
+  Find more information at: https://docs.airshipit.org/airshipctl/
+
 Usage:
   airshipctl [command]
 

cmd/testdata/TestRootGoldenOutput/rootCmd-with-no-subcommands.golden

@@ -1,2 +1,4 @@
 A unified entrypoint to various airship components
 
+  Find more information at: https://docs.airshipit.org/airshipctl/
+

cmd/testdata/TestRootGoldenOutput/specialized-rootCmd-with-bootstrap.golden

@@ -1,5 +1,7 @@
 A unified entrypoint to various airship components
 
+  Find more information at: https://docs.airshipit.org/airshipctl/
+
 Usage:
   airshipctl [command]
 

docs/source/cli/airshipctl.md

@@ -6,6 +6,9 @@ A unified entrypoint to various airship components
 
 A unified entrypoint to various airship components
 
+  Find more information at: https://docs.airshipit.org/airshipctl/
+
+
 ### Options
 
 ```