Document Update: Add BETTER_AUTH_SECRET to Production Environment

The [Convex Better Auth documentation](https://convex-better-auth.netlify.app/#set-environment-variables) instructs users to generate and set the BETTER_AUTH_SECRET key.

It is helpful to remind to also set this key within the Convex production environment.

Start with the linked Convex Better Auth documentation page and locate the section describing BETTER_AUTH_SECRET and environment variables. Update that guidance to mention the Convex production environment, then review the page to confirm the reminder is clear and accurate.

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