docs(billing): document self-serve invoicing details on card update

[SBrandeis]

Users can now optionally provide a business name and invoicing details (e.g. tax ID, PO number) when adding or updating a card, which appear on their invoices. Updated the billing FAQ accordingly.

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Note

Low Risk
Documentation-only change to the billing FAQ with no product or security impact.

Overview
Updates the billing FAQ so users can add a tax ID and invoicing details without emailing support first.

The answer for How can I add my tax ID or update the billing details? now points to https://huggingface.co/settings/billing/payment and the Additional invoicing information (optional) section, where users can enter a Business name and Invoicing details (tax ID, PO number, company registration number) that show on invoices. Support email remains as a fallback for extra help.

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[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Pierrci]

thanks!

[Michellehbn]

TY!