[clang-tools-extra] [clang-doc] Add basic e2e test (PR #93928)

[Paul Kirth via cfe-commits]

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@@ -0,0 +1,358 @@
+// RUN: rm -rf %t && mkdir -p %t/docs && mkdir -p %t/build
----------------
ilovepi wrote:

```suggestion
// RUN: rm -rf %t && mkdir -p %t/docs  %t/build
```
you don't need to run `mkdir` twice.

https://github.com/llvm/llvm-project/pull/93928