docs: document CustomTitleName in Connector API Customer objects (GX-31328)

[ernestfolch-mews]

Summary

Documents the new CustomTitleName property of the Connector API Customer object — the name of the customer's custom title, as configured in the chain's guest titles (Jira: GX-31328, epic GX-31323).

• Customers page: CustomTitleName row added to the Customer object tables (shared object, Add customer response, Update customer response) and to the JSON examples; the adjacent Title rows now state the mutual exclusivity between the two properties, matching the API reference.
• Accounts page: same property documented on the account's Customer object returned by Update accounts (implemented in MewsSystems/mews#87270).
• Changelog: one entry listing all affected operations — Get all/Search/Add/Update customers, Get all reservations (ver 2017-04-12), Get all companionships, customer exports, and Update accounts.

⚠️ Draft until the feature is live in production (rollout tracked in GX-31329). The changelog date 2026-08-31 is a placeholder — it must be set to the actual publish date before merging.

Implementation PRs: MewsSystems/mews#86643, MewsSystems/mews#86892, MewsSystems/mews#87270.

Checklist

• Documentation follows the contribution guidelines
• Changelog accurately describes all changes — pending: placeholder date 2026-08-31 to be set to the actual publish date before merge
• All hyperlinks tested
• SUMMARY.md updated if new pages added (no new pages)

API

• Changelog highlights the affected endpoints or operations
• Changelog highlights any deprecations (none)
• Deprecation Table updated if any deprecations (none)

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