add plotly plot right to landing page

[maxheld83]

actually I think it'd be nice to have a plotly plot of oa-coverage right on the landing page.

Should now be possible programmatically with https://github.com/subugoe/hoad/commit/cc2cd4850160a2b4aaeb01368d7863b472629c95.

So people get an idea without the dashboard loading etc.

Not super interesting for technical users, but crucial for non-technical folk, otherwise looks too technical.