Add maximum width restriction for Markdown Preview

[devleejb]

Description:
Currently, the Markdown Preview does not have a maximum width restriction, causing it to appear excessively wide on large monitors. To enhance the user experience, it is proposed to implement a maximum width limit for the Markdown Preview.

Why:
Setting a maximum width will prevent the content from becoming overly stretched on wide screens, thus improving readability and making the interface more visually appealing. This adjustment will ensure that users have a comfortable viewing experience regardless of their monitor size.

[choidabom]

@devleejb I think it would be better to add a feature that allows the width to be adjusted, similar to how Notion handles it.