Made Markdown wrapper overrideable

[MikeKingdom]

What does this PR do?

Currently any wrapper specified in the Markdown options parameter gets ignored since Markdown forces it to be a Fragment. This change will honor a wrapper specified in options, if any.

Where should the reviewer start?

Markdown.js

What testing has been done on this PR?

A custom storybook example, plus a Jest case added

How should this be manually tested?

const Wrapper = props => <Box gap="small" {...props} />;
...
<Markdown
  options={{ wrapper: Wrapper }}
  components={{
    p: { 
      component: Paragraph,
      props: { margin: 'none' },
    },
  }}
>
  {'first paragraph\n\nsecond paragraph'}
</Markdown>

Do Jest tests follow these best practices?

• screen is used for querying.
• The correct query is used. (Refer to this list of queries)
• userEvent is used in place of fireEvent.
• asFragment() is used for snapshot testing.

Any background context you want to provide?

What are the relevant issues?

#5832

Screenshots (if appropriate)

Do the grommet docs need to be updated?

No

Should this PR be mentioned in the release notes?

Yes

Is this change backwards compatible or is it a breaking change?

Backwards compatible