kernel-intro-sources: add elixir reference

[arnout]

This reference exists in the embedded system development slides, but not in the kernel training. Elixir is kind of important if you ask me :-)

While using the bootlin materials for an internal training, there are a few improvements I made (at least, I consider them improvments myself). Since you probably don't agree with all of them, I'm making separate PRs for each. Feel free to close the PR if you disagree with the idea.

[miquelraynal]

I think I was the one who removed the slide in the first place. There are a number of reasons why I considered removing this:

• I believe if you are down to learning how the kernel works internally, you already are familiar with some text editor. And the text editor mostly does not matter.
• We have URLs towards Elixir all over the place.
• We already give the "take whatever editor" advise in the labs (one of the first labs) and we even advise to use Elixir in one of the vert first labs.

And most importantly, I wanted to reduce the time before people were getting into some real coding, which is usually a part that is enjoyed by trainees, as the introduction was a bit too long.

I don't have any strong feeling either, so I'll let others comment :-)