Fix broken links to Billing

[itsjerryokolo]

Replace all four broken https://thegraph.com/studio/subgraphs/billing/ links in the Subgraph Studio overview with https://thegraph.com/studio/billing/. The old URL returns 404; the new URL opens the Billing page.

The affected steps cover credit-card payments, GRT deposits, GRT withdrawals, and multisig deposits. A repository-wide scan found no other copies of the obsolete URL. The current docs content and supported locales are English-only, so no translation files need changes.

Validation:

• git diff --check passed.
• Verified that exactly four URLs changed and all other file content is unchanged.
• Scanned all 542 tracked files and confirmed no obsolete URLs or escaped/encoded path variants remain.
• Full lint/build was not run locally for this URL-only documentation change; repository CI will run on the PR.

[brandonleekramer]

Looks good, thanks for catching