This shortcode displays a leaderboard of users based on their referral count. The following attributes can be used:
– `exclude_usernames`: A comma-separated list of usernames to exclude from the leaderboard.
– `time_period`: Defines the time period for counting referrals. Accepted values are “lifetime”, “year”, “month”, or “day” (default is “lifetime”).
– `limit`: Sets the maximum number of users to display on the leaderboard (default is 10).