[Feature Request] Better documentation and config for column renaming

[lonix1]

Steps: - delete the `vscode-kanban.json` - open kanban tab, which recreates that file - close kanban tab - add the [renaming stuff](https://github.com/mkloubert/vscode-kanban#columns-) to the file, so the file looks like below - reopen kanban tab - columns are not renamed (and sometimes a little loading indicator animation is shown in each column, which carries on forever)

{
  "todo": [],
  "in-progress": [],
  "testing": [],
  "done": [],
  "kanban": {
    "columns": {
      "done": "Finished",
      "inProgress": "Work",
      "testing": "Currently Testing",
      "todo": "Ideas and TODOs"
    }
  }
}

[lonix1]

Okay turns out the column renaming stuff should be in the workspace config (settings.json), not in the extension's config file (vscode-kanban.json).

Should be mentioned in the docs.

(Also, should be possible to have it in vscode-kanban.json instead. Putting config in the workspace config file is for extensions that don't have their own config file - but this one does.)