[llvm] [Github] Enable warnings as errors on flang sphinx build (PR #72723)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

We'll probably need to wait a little bit before merging this, but there's only one final warning that I can see before this is ready to merge:
```
/tmp/llvm-project/build/tools/flang/docs/Source/FlangCommandLineReference.rst:176: WARNING: unknown document: 'DiagnosticsReference'
```

Once that gets fixed, we should be able to get this in.

https://github.com/llvm/llvm-project/pull/72723