feat(invoice): Document refactoring on invoice amounts

[vincent-pochet]

Update Invoice api ref documentation:

New fields

• currency
• fees_amount_cents
• coupons_amount_cents
• credit_notes_amount_cents
• sub_total_vat_excluded_amount_cents
• sub_total_vat_included_amount_cents
• prepaid_credit_amount_cents

Deprecated fields

• legacy
• amount_currency
• vat_amount_currency
• credit_amount_currency
• total_amount_currency
• amount_cents
• credit_amount_cents