Add subscription event types to X Activity API docs

[mintlify]

Documents the new subscriptions.subscribe and subscriptions.unsubscribe event types for the X Activity API.

• Introduction: new "Subscription events" section under Follow events with event table, direction qualifier support, and a private-events note (follows.read scope). Also notes direction qualifier support on Follow events, adds both subscription events to the filter qualifiers table, and lists them under private events.
• Event payloads: adds sample subscriptions.subscribe and subscriptions.unsubscribe delivery payloads.

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Requested by: mrosinsky@x.ai via Slack
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[mintlify]

Docs PR opened: #403

Added a Sep 9 changelog entry covering new X Activity API subscription events and the broadcast.read/broadcast.write OAuth scopes.