Write documentation for each CI workflow

[BD103]

While I was working on #13033, I noticed that the workflows in CI are a bit difficult to understand for beginners. To amend this, I added short descriptions to the beginning of all actions in .github/workflows summarizing what they do.

r? @blyxyas

(I'm requesting your review because it's related to #13033, and I also don't know anyone else who's familiar with the CI in this project. Please point me in the right direction if there's someone better suited for this PR!)

changelog: none

[rustbot]

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[blyxyas]

Thanks for the documentation changes! Just a couple nits and this should be merged. ❤️

[blyxyas]

I think it would be great if we specified which steps are taking for checking the project as a whole.

Suggested change

	# This action runs tests specific to `clippy_dev`, accessing through the `cargo dev` command. It
	# both checks the project as a whole and tests `clippy_dev`'s commands specifically.
	# This action runs tests specific to `clippy_dev`, accessing through the `cargo dev` command. It
	# both checks the project as a whole (formatting & updating the lints) and `clippy_dev`'s commands specifically.

[blyxyas]

Suggested change

	# This action runs Clippy's test suite for all of its crates.
	# This action runs Clippy's test suite on the pull request patch.
	# Making sure that a pull request doesn't break anything.

[blyxyas]

Usually it's @flip1995 the go-to person for CI changes, so he could give better advice than me, but let me know if you're stuck at any point in the process!

[flip1995]

Hmm, I would prefer to write documentation about the CI setup in the book. This is also one of my forever TODOs...

Maybe this gives me the necessary motivation to do this over the weekend. If not, we can merge this.

[BD103]

@flip1995 would you like me to move the notes into the Clippy book instead? If you tell me where you want it, I can easily move it over. :)

[flip1995]

Nah the notes are not extensive enough to paint the full picture of how our CI setup works and why each workflow exists. I really need to write down the status quo of CI. I had to take a break from my computer this weekend though...