Update source.md according to Filesystem Hierarchy Standard

[cretingame]

According to the Filesystem Hierarchy Standard (FHS), /usr/local is specifically intended for software manually installed by the system administrator (i.e., outside the distribution's package manager), which is the case for the clang + LLVM 17 tarball.

Why /usr/local is preferred over /usr

• Avoids conflicts: /usr is reserved for distribution-managed software. Installing to /usr might overwrite files from system packages or be overwritten in future package updates.
• Cleaner management: Tools like checkinstall or system backups can more easily identify "local" vs. "distro" files.
• Conformance: Installing to /usr/local conforms to Unix and Linux conventions and best practices.

[8bitmp3]

@MarkDaoust @markmcd