[llvm][Docs] Update MyFirstTypoFix doc (#79149)

I've not tried to change the purpose or style of the doc, just edited
for clarity and removed any Phabricator related language in favour of
GitHub terms.

Where possible, I've swapped direct links to LLVM's website with RST
links to the local documents. Which should be a bit more resilient.

Also it's less confusing if you're editing multiple pages locally, you
don't accidentally end up on the live site.