Add /2/usage/credits endpoint to Usage docs

[mintlify]

Adds the new GET /2/usage/credits endpoint (PPU credit balance in USD) to the Usage section.

Changes

• docs.json: added x-api/usage/get-usage-credits to the Usage group so the auto-generated page is reachable in the sidebar.
• x-api/usage/introduction.mdx: added the new endpoint to the endpoints table and updated the section description to mention PPU credits.

The MDX page itself (x-api/usage/get-usage-credits.mdx) was generated by the Update Documentation Action run against the latest api.x.com/2/openapi.json.

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Requested by: pvann@x.ai via Slack
Mintlify session: slack_1787596117.503339_C09MQDT6BM4

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[mintlify]

Docs PR opened: #330

Added an Aug 24 changelog entry announcing the new GET /2/usage/credits endpoint for checking Pay-per-use credit balances.