[KSAGlory]

Document how Zstandard's Makefile controls installation paths. This clarifies the /usr/local default, the PREFIX override, and DESTDIR staging so users can customize make install without inspecting the Makefiles.

Closes #4757.

Testing:

• Ran git diff --check.
• Ran make -n install PREFIX=/opt/zstd DESTDIR=/tmp/zstd-package.
• Confirmed the dry run targets /tmp/zstd-package/opt/zstd for binaries, headers, and libraries.
• Fork validation passed 103 of 104 checks. Android NDK Build fails before compilation because the upstream Setup Android SDK action invokes sdkmanager tools, which reports Failed to find package 'tools'. The failed job was retried twice with the same result.