[docs] Fix some finicky Sphinx warnings.
[docs] Fix Sphinx toctree warning.

Last I heard LSan is getting close to prime-time, but for now keep
LeakSanitizer.rst hidden (especially since it contains so little
content).
[docs] End users should see just "Clang"

Half the internet has wildly incorrect ideas about what LLVM is (in
particular, many get the impression that LLVM is some magical
cross-platform runtime), so saying "LLVM" near "cross-compilation" in a
user-facing doc might provoke confusion.