utils, help: Show basic help when "man" command is not available. Fixes

containers#713

Fedora CoreOS systems do not have the man command installed. Running
toolbox --help on such a system results in a "man(1) not found" error.

As a compromise for systems without man, we added a simple help text
showing the most commonly used toolbox commands and an URL that direct
users to the Toolbox website where they can find the manuals in Markdown
format.

src/pkg/utils/utils.go

@@ -731,7 +731,18 @@ func ShowManual(manual string) error {
 	manBinary, err := exec.LookPath("man")
 	if err != nil {
 		if errors.Is(err, exec.ErrNotFound) {
-			return errors.New("man(1) not found")
+			fmt.Print(`
+toolbox - Tool for containerized command line environments on Linux
+
+Common commands are:
+create    Create a new toolbox container
+enter     Enter an existing toolbox container
+list      List all existing toolbox containers and images
+
+Go to https://github.com/containers/toolbox for further information.
+
+`)
+			return nil
 		}
 
 		return errors.New("failed to lookup man(1)")