[clang] [Sanitizer][Docs] Improve docs on building Asan (PR #68636)

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

I am to late with comment, can you make it in the same style as https://llvm.org/docs/CMake.html

One line, and with $:
```
.. code-block:: console

  $  cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="clang"  -DLLVM_ENABLE_RUNTIMES="compiler-rt" path/to/llvm/source
```

https://github.com/llvm/llvm-project/pull/68636