Update CONTRIBUTING.md

[eviau-artefactual]

This update should reflect changes to the process.

It also contains some rephrasing, for concision.

Suggestions are welcome on how this could be further improved.
As an example, I find this paragraph to be a bit... tedious?

If nothing comes up, start a conversation by opening a new thread
on the user forum. When your bug will have been better understood,
you may be invited to open an issue on GitHub.

[fiver-watson]

I have added a couple suggestions, for that one sentence but also as general considerations about some of the changes and the links. You may have already considered these things (in which case ignore); for now just ensuring that we are not implying promises we don't intend to keep.

Otherwise, LGTM!

[melaniekung]

hi edith, added a couple comments but otherwise, looks good to me! thank you for addressing this :)

[fiver-watson]

LGTM 👍