add preview:showhiddenfiles to config docs (#1619)

Author: sawka

docs/docs/config.mdx

@@ -50,6 +50,7 @@ wsh editconfig
 | editor:minimapenabled                | bool     | set to false to disable editor minimap                                                                                                                                                                                                                        |
 | editor:stickyscrollenabled           | bool     | enables monaco editor's stickyScroll feature (pinning headers of current context, e.g. class names, method names, etc.), defaults to false                                                                                                                    |
 | editor:wordwrap                      | bool     | set to true to enable word wrapping in the editor (defaults to false)                                                                                                                                                                                         |
+| preview:showhiddenfiles              | bool     | set to false to disable showing hidden files in the directory preview (defaults to true)                                                                                                                                                                      |
 | markdown:fontsize                    | float64  | font size for the normal text when rendering markdown in preview. headers are scaled up from this size, (default 14px)                                                                                                                                        |
 | markdown:fixedfontsize               | float64  | font size for the code blocks when rendering markdown in preview (default is 12px)                                                                                                                                                                            |
 | web:openlinksinternally              | bool     | set to false to open web links in external browser                                                                                                                                                                                                            |