CON-9617: deprecate Add credit card payment

[jnv]

Summary

CON-9617: Deprecate the Add credit card payment operation.

Replaces #33, which was opened before the Payment automation use case was rewritten for Mews Payments Checkout (#68, #69). This PR reapplies the parts of #33 that are still relevant on top of that rewrite:

• Mark Add credit card payment as deprecated, with discontinuation on 10 January 2028.
• Add the changelog entry (dated 29 Jul 2026) and the deprecation table row.
• Strike through the operation in the operations index.
• Remove the operation from the "how to" index and the Kiosk use case.
• Reformat Kiosk to use GitBook hint blocks and reference-style links, as in CON-9617: Deprecate payments/addCreditCard #33.
• Drop the "Charge credit card vs Add credit card payment" hint from Payment automation, since the comparison no longer applies.

Not carried over from #33:

• The rest of the payment-automation.md changes – superseded by the Mews Payments Checkout rewrite.
• The pilot-program wording and the "migration guide in Q2 2026" line in the changelog – Mews Payments Checkout is now documented as generally available.

Checklist

• Documentation follows the contribution guidelines
• Changelog accurately describes all changes
• All hyperlinks tested
• SUMMARY.md updated if new pages added

API

• Changelog highlights the affected endpoints or operations
• Changelog highlights any deprecations
• Deprecation Table updated if any deprecations

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