[clang] [clang/www/get_started.html] Use newer `cmake` syntax (PR #93503)

[Aaron Ballman via cfe-commits]

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@@ -67,15 +67,13 @@ <h3 id="buildNix">On Unix-like Systems</h3>
   <li>Build LLVM and Clang:
   <ul>
     <li><tt>cd llvm-project</tt></li>
-    <li><tt>mkdir build</tt> (in-tree build is not supported)</li>
-    <li><tt>cd build</tt></li>
     <li>This builds both LLVM and Clang in release mode. Alternatively, if
         you need a debug build, switch Release to Debug. See
         <a href="https://llvm.org/docs/CMake.html#frequently-used-cmake-variables">frequently used cmake variables</a>
         for more options.
     </li>
-    <li><tt>cmake -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_BUILD_TYPE=Release -G "Unix Makefiles" ../llvm</tt></li>
----------------
AaronBallman wrote:

I'm wondering what the benefits are to using `-S` and `-B`? Is it purely as shorthand for `cd` and `mkdir` effectively, or does it do more beyond that? (I think we want the docs to be as general as possible and we want to lean on CMake's documentation for bells and whistles.)

https://github.com/llvm/llvm-project/pull/93503