docs: factor large docs into separate .md files to make them easier to maintain.

We then use the go embed command to embed them back into the binary.

vfs/vfs.go

@@ -22,6 +22,7 @@ package vfs
 
 import (
 	"context"
+	_ "embed"
 	"fmt"
 	"io"
 	"os"
@@ -42,6 +43,11 @@ import (
 	"github.com/rclone/rclone/vfs/vfscommon"
 )
 
+// Help for the VFS.
+//
+//go:embed vfs.md
+var Help string
+
 // Node represents either a directory (*Dir) or a file (*File)
 type Node interface {
 	os.FileInfo