[llvm][Docs] Expand MyFirstTypoFix's post-commit section

[DavidSpickett]

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.

[DavidSpickett]

This uses content from #78292, so if/when this lands, I'll update that PR to be a much smaller comment that links to this new documentation.

[asb]

LGTM. Regarding cutting down #78292, it's a tricky balance as there's also advantage to having useful info readable without needing to click-through. But happy to comment on if I agree the balance is right once you have updated that PR. I suspect noting that buildbot failures may occur for reasons other than your patch being bad and linking to this doc for more info on diagnosing that could make sense.