Lib: Change legend font-size

[louismaximepiton]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Set a (optional?) smaller font-size on the legend and the legend in the tooltip as discussed in #88 (comment). It needs to be confirmed and discussed if the used font-size will have a breakpoint or not. For now, what has been decided is to use 14px under the tablet breakpoint (under lg) and 16px above. Once the rule is final it should be mentioned in the DSM (might be done for OUDS only)

Motivation and context

Be more consistent between legend and axis labels.
Take less place while keeping the legibility of the legend.