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

[Vlad Serebrennikov 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>
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Endilll wrote:

Yeah, instead of spreading the complexity between three lines, it's now concentrated in one.
I'd expect users to be more familiar with `cd` and `mkdir` than with `-S` and `-B`, so this change would make the docs less accessible.

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