docs: clarify pay by invoice setup and availability

[Sudaisib]

Description

This PR clarifies the pay-by-invoice guidance in the payment methods documentation.

What changed

• Reworded the pay-by-invoice section to make the setup flow clearer
• Clarified that Docker Sales may need to enable pay by invoice before it appears in Docker Home
• Removed the duplicate pay-by-invoice section at the bottom of the page

Why

The page contained confusing guidance about who can enable pay by invoice and when the option is available.

Validation

• Reviewed the wording for consistency with related billing docs
• Confirmed the updated text uses simpler wording and avoids the previously flagged term

Fixes #26024

Reviews

• Technical review
• Editorial review
• Product review

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