message: docs: Add a table for visibility values in our JSON Schema
diff: @@ -83,6 +83,12 @@ object will be available in the frontend. The full ancestry does not need to
  have correctly defined visibilities however, so it is enough to only for example
  declare the visibility of a leaf node of `type: "string"`.
  +| `visibility` | |
  +| ------------ | ------------------------------------------------------------------ |
  +| `frontend` | Visible in frontend and backend |
  +| `backend` | (Default) Only in backend |
  +| `secret` | Only in backend and may be excluded from logs for security reasons |
  +
  ## Validation
  Schemas can be validated using the `backstage-cli config:check` command. If you
  