Improve contributing guide

[gwarf]

Improve contributing guide, adding some explicit documentation on using git and GitHub CLI.

[enolfc]

Should we move this to the actual page instead of linking?

[enolfc]

Thanks @gwarf ! Some minor comments.

I feel the style of docs should be highlighted before all the PR details (there is only the comment on valid MD and 80 lines)

[gwarf]

I've disabled prettier for now, in fact it's more to be used on users' computers to re-write the content properly than to be used as a linter as the --check output is not very convenient/meaningful: it can only show that files have not went through prettier.
This should still be enough to be used to check code, but will only be enabled when most/all content will have been properly updated.

See https://prettier.io/docs/en/comparison.html

[gwarf]

I've also started to add a place with some markdown tips, would be good to have us add things that make sense and that we easily forgot. I was lazy and put it in the style page...

[enolfc]

Go!