docs(mac): Add documentation for -system-libs build mode

[gaurav02081]

[IMPROVEMENT]

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PR Summary

This PR documents the macOS -system-libs build mode introduced in #1862, which enables building CCExtractor using system-installed libraries instead of bundled dependencies.

This mode is required for Homebrew compatibility and uses pkg-config to link against Homebrew-provided libraries (e.g. gpac, freetype, libpng, zlib). The default bundled build remains unchanged.

The documentation explains:

When to use the default vs -system-libs build

Required Homebrew dependencies

How to verify the build

CI coverage for the system-libs path

This provides a clear upstream reference for package managers and unblocks future Homebrew formula work.

[x15sr71]

Thanks for the PR @gaurav-dev02, this documentation is comprehensive and addresses the Homebrew compatibility clearly.
I had a couple of structural suggestions from a maintainability point of view -

1. Since docs/compilation.md is currently the main entry point for build instructions, i think it might be cleaner to add this section directly there (under macOS) instead of a new standalone file? It might help users find all build options in one place.
2. Some of the background/history sections could potentially be trimmed to keep the focus on the “how to build” path.

Thanks for unblocking the docs side!