docs: fix broken "Control your costs" link in billing overview (#42906)

## What kind of change does this PR introduce?

Documentation fix

## What is the current behavior?

In `apps/docs/content/guides/platform/billing-on-supabase.mdx`, the
"Control your costs" link has an empty URL (`[Control your costs]()`),
making it non-functional. Users clicking this link are not taken to the
cost control documentation.

## What is the new behavior?

The link now correctly points to `/docs/guides/platform/cost-control`,
which is the existing page that covers Spend Cap, Billing Alerts, and
other cost control features.

## Additional context

The other two links in the same list (`Your monthly invoice` and `Manage
your usage`) already have correct URLs. This appears to be an oversight
where the cost-control page was created but the link in the billing
overview was not updated.

apps/docs/content/guides/platform/billing-on-supabase.mdx

@@ -39,7 +39,7 @@ Monthly costs for paid plans include a fixed subscription fee based on your chos
 
 - [Your monthly invoice](/docs/guides/platform/your-monthly-invoice) - For a detailed breakdown of what a monthly invoice includes
 - [Manage your usage](/docs/guides/platform/manage-your-usage) - For details on how the different usage items are billed, and how to optimize usage and reduce costs
-- [Control your costs]() - For details on how you can control your costs in case unexpected high usage occurs
+- [Control your costs](/docs/guides/platform/cost-control) - For details on how you can control your costs in case unexpected high usage occurs
 
 ### Compute costs for projects