📝 (doc) Slight improvement of README.md

[xibe]

Purpose

I tried to make the README file a bit better and easier to read for newcomers, by rephrasing a few sentences, moving things around, and in general trying to make it clearer.

As a next step, we would certainly benefit from re-using the copy on the https://docs.numerique.gouv.fr/ site -- both because it is great, and because it would bring a consistent message throughout.

So please consider this a first pass :)

Proposed changes

• Rewrote the headline.
• Updated and reorganize the Write/Collaborate/Self-host sections, trying especially to remove negative wording. Also provided link to Markdown syntax, for those who don't know it.
• Provided a more permanent link for the test environment. Already fixed in 🐛(readme) fix preprod link to redirect to homepage #747
• Made typographic fixes here and there.

[virgile-dev]

@xibe saw you closing this. Are you planning to open another PR?
Let me know, if you are short on time I can create one on my end.

[xibe]

@virgile-dev My git-fu is rusty, and because I created this PR entirely in GitHub I couldn't make head or tails for rebasing it 😅
So I choose the easy way out, and redid the whole thing locally then pushed it to GitHub, like grown-ups do.
Feel free to inspect #848 ! :)