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

[Mariusz Sikora via cfe-commits]

mariusz-sikora-at-amd 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
```

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