[Multipage docs] Make sure that the thumbnail and the title are displayed one below the other

[brauliodiez]

Right now depending on the resolution title maybe be shown on the left side

Just use some proper flex container.

Current buggy behavior (wide screen monitor) Page should be always shown below the thumbnail and centered

Seems that we need some flex container on it, file to check:

src/pods/thumb-pages/thumb-pages.pod.tsx

  return (
    <div className={classes.container}>
      {fullDocument.pages.map((page, index) => (
        <React.Fragment key={page.id}>
          <ThumbPage pageIndex={index} onSetActivePage={handleSetActivePage} />
          <div>{page.name}</div>
        </React.Fragment>
      ))}
      <button onClick={handleAddNewPage}>Add new page</button>
    </div>
  );