Update docs for Session Replay start/stop/pause/resume APIs

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Goal

Document the new public Session Replay APIs for Android, matching the existing JavaScript Session Replay docs.

What to document

• Sentry.replay().start() — starts a session-mode replay
• Sentry.replay().startBuffering() — starts a buffer-mode replay (rolling window, flushed on error or manual flush())
• Sentry.replay().stop() — stops recording, stop() → start() begins a new session
• Sentry.replay().pause() — pauses recording, blocks auto-resume until resume() is called
• Sentry.replay().resume() — resumes a paused recording
• Sentry.replay().flush() — flushes the buffer and sends it
• Setting both sample rates to 0 and calling start() / startBuffering() manually
• All methods are safe to call from any thread

Where

• Session Replay setup page — add a "Manual Control" section
• Session Replay configuration page — mention the sample-rate-0 + manual start pattern
• Sentry Changelog entry

Prerequisites

• JAVA-325 (public API) must be merged first

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JAVA-691