docs: add weekly changelog entry for paid_partnership Post field

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Summary

Polishes this week's changelog entry covering the paid_partnership Post field — the only product change shipped since the last entry (May 4, 2026).

Changes

• Restructured the Jun 3, 2026 changelog entry under a New features heading so it follows the standard new features → updates → bug fixes order.
• Added a short overview describing what the change means for users (authors can disclose paid promotional content; X labels the Post accordingly).
• Documented both the create-side (POST /2/tweets with paid_partnership: true) and read-side (tweet.fields=paid_partnership) usage as skimmable bullets.
• Linked to the Post fields data dictionary and Manage Posts quickstart for deeper reference.

Context

Based on the merged change docs: add paid_partnership tweet field (commit 9338af0), which is the only product update since the May 4, 2026 search-index migration entry.

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