Apply style guide: X Activity API introduction

[mintlify]

Summary

Applies style guide consistency fixes to x-api/activity/introduction.mdx.

Changes

• Rewrote intro paragraph to use second person ("you") and active voice, and shortened a 32-word sentence.
• Fixed title-case "Enterprise Only" heading label to sentence case ("Enterprise only") to match other Note labels.
• Added missing possessive apostrophe in the dm.read table row ("filtered user's").
• Tightened awkward phrasing in the "Event privacy and authentication" intro.
• Changed "A User has to authenticate" to "A user must authenticate" for consistent capitalization and directness.

Context

Follow-up to #246, which added new private events and updated subscription limits.

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