[llvm][Docs] Expand MyFirstTypoFix's post-commit section (#79827)

And link to it from the GitHub guide. Since it fits in the flow of the
former document better, but is of interest to those using the GitHub
specific guide.

This includes some content I wrote for an automated PR comment but I
think is useful to have in a single place to link to instead.

I have removed some details:
* About the waterfall view because it doesn't seem that useful to new
contributors and is likely to just seem like constant chaos given how
many builds are going on.
* The orange bubbles in the console view. I also remember this being the
case but whether it was changed, or the web UI is just not loading
properly, I now see red builds that were red before that commit. So
again, it seems like detail that's not needed for a new contributor.