[Ky0-Nguyen]

Adds a short step to the Android system tracing guide in docs/profiling.md: check that a trace actually contains frames before reading it.

A capture with no frame data is easy to read as a smooth app, because an empty track looks the same as a fast one. The new step:

• lists what to check for the recorded interaction: the app's process, Choreographer#doFrame / DrawFrame slices, and on Android 12+ the Expected/Actual Timeline tracks;
• gives the common reasons a trace has no frames (not run as profileable, recording missed the interaction, device doesn't report the tracks);
• states the frame budget by refresh rate (16.7ms at 60Hz, 11.1ms at 90Hz, 8.3ms at 120Hz) rather than a fixed 16ms.

The following steps are renumbered (Reading the trace is now 3, Find your process is 4). Nothing else on the page links to those anchors.

Context: react-native-community/discussions-and-proposals#1025. Thanks to @SanketPathare for suggesting that the docs define what counts as a frame and which threshold applies.

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