[llvm] [Github][CI] Add `doc8` for clang-tidy documentation formatting (PR #168827)

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``````````bash
darker --check --diff -r origin/main...HEAD llvm/utils/git/code-lint-helper.py
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``````````diff
--- code-lint-helper.py	2025-11-20 06:08:36.000000 +0000
+++ code-lint-helper.py	2025-11-20 06:12:16.146033 +0000
@@ -231,11 +231,10 @@
         text=True,
         check=False,
     )
 
     return clean_clang_tidy_output(proc.stdout.strip())
-
 
 
 def clean_doc8_output(output: str) -> Optional[str]:
     if not output:
         return None

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https://github.com/llvm/llvm-project/pull/168827