[clang] [docs][clang-format] Rename clang-format docs *.rst -> *.md, update refs (PR #211397)

[via cfe-commits]

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@@ -67,7 +67,7 @@ set(docs_tools_dir ${CLANG_SOURCE_DIR}/docs/tools)
 
 set(format_style_depend ${CMAKE_CURRENT_BINARY_DIR}/format_style_depend)
 set(dump_style dump_format_style.py)
-set(style_options_rst ${CLANG_SOURCE_DIR}/docs/ClangFormatStyleOptions.rst)
+set(style_options_rst ${CLANG_SOURCE_DIR}/docs/ClangFormatStyleOptions.md)
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owenca wrote:

```suggestion
set(style_options_md ${CLANG_SOURCE_DIR}/docs/ClangFormatStyleOptions.md)
```
For consistency, IMO.

https://github.com/llvm/llvm-project/pull/211397