[llvm] [Doc] Add a section on CI to the GitHub documentation (PR #85376)

[Nikita Popov via llvm-commits]

================
@@ -176,6 +176,24 @@ request will understand that you're rebasing just your patches, and display
 this result correctly with a note that a force push did occur.
 
 
+Pre-merge Continuous Integration (CI)
+-------------------------------------
+
+Multiple checks will be applies on a pull-request, either for linting/formatting
----------------
nikic wrote:

```suggestion
Multiple checks will be applied on a pull-request, either for linting/formatting
```

https://github.com/llvm/llvm-project/pull/85376