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

[Nikita Popov via llvm-commits]

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@@ -124,12 +124,23 @@ For developers to commit changes from Git
 -----------------------------------------
 
 Once a patch is reviewed, you can select the "Squash and merge" button in the
-GitHub web interface. You might need to rebase your change before pushing
-it to the repo.
+GitHub web interface.
 
-LLVM currently has a linear-history policy, which means that merge commits are
-not allowed. The `llvm-project` repo on github is configured to reject pushes
-that include merges, so the `git rebase` step above is required.
+When pushing directly from the command-line to the `main`` branch, you will need
+to rebase your change. LLVM currently has a linear-history policy, which means
+that merge commits are not allowed and the `main` branch is configured to reject
+pushes that include merges.
+GitHub will display a message that looks like:
----------------
nikic wrote:

Add an empty line above here? (Not sure about how rst interprets this.)

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