docs: replace Owned Reads card with X Chat API launch on overview

[mintlify]

Replaces the "Pay Per Use Pricing Changes: Featuring Owned Reads" card on the overview page with an "X Chat API launched" announcement:

• Updated title, copy, and Learn more link
• Swapped the faded X logo background for the xds icon-xchat.svg chat icon
• Moved the card up into the Products section (right below the pay-per-use tip)

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Generated by Mintlify Agent.

Requested by: smedinarolong@x.com via Slack
Mintlify session: slack_1785539953.068399_C09MQDT6BM4

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