fix(linux): add missing /opt directory explanation

src/data/roadmaps/linux/content/navigation-basics/directory-hierarchy.md

@@ -11,6 +11,7 @@ In Linux, understanding the directory hierarchy is crucial for efficient navigat
 - `/usr`: User programs and data.
 - `/lib`: Shared libraries.
 - `/tmp`: Temporary files.
+- `/opt`: Third-party applications.
 
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