Fixes some typos in the documentation

To make it 100% clear that the changes are an example and the guide
should later be used with the reader's own changes to submit an
actual PR.

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.

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.

Fix various formatting issues in MyFirstTypoFix.

This adds first version of a GitHub workflow in the documentation and marks some
sections as deprecated. We should clean up these sections ASAP. I was
just keen to get something on the documentation site as soon as
possible.