powermetrics: add page

[nil957]

Adds a tldr page for powermetrics, a macOS utility for displaying CPU usage, power consumption, and other system metrics.

Closes #21876

About powermetrics

powermetrics is a pre-installed macOS utility that provides power-related measurements including:

• CPU usage statistics (user mode and supervisor mode)
• Timer and interrupt wakeup frequency
• Package C-state statistics
• Estimated power consumption by SoC subsystems (CPU, GPU, ANE)
• Per-process energy impact

Examples included

• Display power metrics with default settings
• Sample at a specific interval
• Take a specific number of samples
• Show per-process energy impact
• Output in plist format for machine parsing
• Save output to a file
• Use specific samplers
• Display all available information

More information: https://www.unix.com/man_page/osx/1/powermetrics/

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