docs: add a new page with global flags and link to it from the command docs

In f544234 we removed the global flags from each command as it was
making each page very big and causing 1000s of lines of duplication in
the man page.

This change adds a new flags page with all the global flags on and
links each command page to it.

Fixes #3273

cmd/help.go

@@ -33,6 +33,7 @@ documentation, changelog and configuration walkthroughs.
 		atexit.Run()
 	},
 	BashCompletionFunction: bashCompletionFunc,
+	DisableAutoGenTag:      true,
 }
 
 const (
@@ -74,6 +75,10 @@ __rclone_custom_func() {
 `
 )
 
+// GeneratingDocs is set by rclone gendocs to alter the format of the
+// output suitable for the documentation.
+var GeneratingDocs = false
+
 // root help command
 var helpCommand = &cobra.Command{
 	Use:   "help",
@@ -100,7 +105,11 @@ var helpFlags = &cobra.Command{
 			}
 			flagsRe = re
 		}
-		Root.SetOutput(os.Stdout)
+		if GeneratingDocs {
+			Root.SetUsageTemplate(docFlagsTemplate)
+		} else {
+			Root.SetOutput(os.Stdout)
+		}
 		_ = command.Usage()
 	},
 }
@@ -222,6 +231,35 @@ Use "rclone help flags" for to see the global flags.
 Use "rclone help backends" for a list of supported services.
 `
 
+var docFlagsTemplate = `---
+title: "Global Flags"
+description: "Rclone Global Flags"
+date: "YYYY-MM-DD"
+---
+
+# Global Flags
+
+This describes the global flags available to every rclone command
+split into two groups, non backend and backend flags.
+
+## Non Backend Flags
+
+These flags are available for every command.
+
+` + "```" + `
+{{(backendFlags . false).FlagUsages | trimTrailingWhitespaces}}
+` + "```" + `
+
+## Backend Flags
+
+These flags are available for every command. They control the backends
+and may be set in the config file.
+
+` + "```" + `
+{{(backendFlags . true).FlagUsages | trimTrailingWhitespaces}}
+` + "```" + `
+`
+
 // show all the backends
 func showBackends() {
 	fmt.Printf("All rclone backends:\n\n")