Add some documentation about chaarts usage

[nhoizey]

Even if it's not yet usable as an installable library (#27), it currently requires a bit of digging to understand how to build graphs like on the demo site.

For example, for a bar chart:

• load https://github.com/ffoodd/chaarts/blob/main/dist/chaarts.css , not just the CSS from the example
• it looks like part (or all?) of the example styles are already inside chaarts.css
• add a container element with class chaarts-container, <table class="chaarts bar"…> is not enough
• add data-theme="dark" to the html element to get the dark theme (at least if there is no prefers-color-scheme management on the site?)