docs(billing): point invoice email prefs to Subscriptions tab

[HaoChiBao]

Summary

• Point "Turn on invoice emails" to Subscriptions → Billing Email (not Invoices and documents).
• Align the billing dashboard overview so invoice email preferences are listed under Subscriptions.

Fixes #31128.

Why

Users cannot find the control where the docs send them. Closed PR #31608 already had the correct path but was auto-closed for staleness without merge. Download-invoice steps stay on Invoices and documents — only the email opt-in path changes.

Verification

• Matches the path from closed Billing: correct location of invoice email preferences (Subscriptions tab) #31608 and reporter confirmation that preferences live on the Subscriptions Billing Email card.
• Diff limited to the opt-in steps + one overview bullet; download flow untouched.

Test plan

• On a non-Enterprise account: Billing → Subscriptions → Billing Email shows invoice email opt-in
• Download invoice section still says Invoices and documents

[cloudflare-docs-bot]

Review

✅ No issues found in commit 85b9ecf.

Code Review

This code review is in beta and may not always be helpful — use your judgment.

_{No code review issues found.}

Conventions

_{No convention issues found.}

Style Guide Review

_{No style-guide issues found.}

Commands

Only codeowners can run commands. Post a comment with the command to trigger it.

Command	Description
/review	Runs a review now. Incremental if a prior review exists, full if not.
/full-review	Re-reviews the entire PR diff from scratch, ignoring incremental history. Useful after a rebase, when you want a fresh review, or if the bot gets out of sync and reports issues that no longer exist.
/ignore-review-limit	Permanently lifts the 2-review automatic limit for this PR. Future pushes will trigger reviews as normal.
/disable-auto-review	Stops automatic reviews from triggering on future pushes to this PR. Codeowners can still run /review or /full-review manually.
/rebase	Rebases the PR branch against production. On conflict, attempts to resolve automatically using AI. Stops with an explanation if confidence is not high enough.