docs: Improve contributing

[MarieSacksick]

Closes #882

Add

• say where people can get the logo --> I suggest we don't do it because there is no reason to add logos now.
• say that images on README should not break in pypi --> cancelled for now as I don't see any constraint in their doc.
• where to store large files
• what files can be modified when tackling examples
• say how to run the tests (whether only back or front)
• for the UI, say sometimes must do hard refresh -> UI, dark mode: the name of items doesn't appear well #1011 (comment)
• add a code of conduct

[MarieSacksick]

say that images on README should not break in pypi

I didn't find any constraint specified in pypi doc, therefore I don't plan to add anything.

[github-actions]

Documentation preview @ a2a352d

[auguste-probabl]

Great work, thanks! The "values" section is especially welcome. I added a few suggestions

[auguste-probabl]

Thanks!