feat: add blurb about account balances

[IanKrieger]

No description provided.

[lukaslevert]

Here's my suggested edits for clarity/consistency with rest of docs:

Account balances

Any leftover campaign budgets from pre-paid campaigns paid via Stripe or Radom will be transferred to your account balance for future campaigns. These funds are typically transferred within 24-48 hours, under the following two circumstances:

When a campaign reaches its end date

If your campaign reaches its scheduled end date and there are remaining funds, the campaign status will update to "complete," and any leftover funds will be transferred to your account balance.

When a campaign is terminated manually

To end your campaign immediately and transfer any remaining funds to your account balance, follow these steps:

• Select the campaign.
• Click "Edit."
• Click "Close this campaign."