[clang] [clang][CIR][doc] Add auto-generated ClangIR documentation (PR #190354)

[Sirui Mu via cfe-commits]

Lancern wrote:

> Hi @Lancern, I see (locally) the following error after this change with Python 3.8.
> 
> ```
> 
> docs/CIR/_raw/PostProcessCIRDocs.py", line 45
> 
>     index_content += f"""
> 
>                      ^
> 
> SyntaxError: f-string expression part cannot include a backslash
> 
> ```

Ooops... obviously f-string is not available in Python 3.8 . Do we have a requirement on the minimal Python version we have to support?

https://github.com/llvm/llvm-project/pull/190354