Add dismissible attribute to Box

[alyip98]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)
• [x] Enhancement to an existing feature

Related PR

What is the rationale for this request?
We can enhance the Box feature to allow the boxes to be closed.

What changes did you make? (Give an overview)
Added a dismissible attribute to <box> that adds a close button to the Box.

Provide some example code that this change will affect:

<box type="info" dismissible>
Info box
</box>

Is there anything you'd like reviewers to focus on?

Testing instructions:
The user guide section in the netlify preview contains an example that demonstrates the dismissible attribute.

Proposed commit message: (wrap lines at 72 characters)
Let's enhance the Box feature by adding support for a dismissible
attribute, which adds a close button to the box.

[Chng-Zhi-Xuan]

Code looks fine.

However, your commit message did not wrap at 72 characters.