Link to dbc and driver manager install instructions at top of docs home page

[ianmcook]

After we add a guide showing how to the ADBC install driver manager for your language (#120), let's update the top of the home page to look something like this:

(The only thing that I changed in this mockup is the text above each of the code boxes.)

[ianmcook]

Alternatively, we could add the full installation instructions for dbc and the driver managers right at the top of the page. But I don't like that, because it distracts from the simplicity of the user experience which this above-the-fold section on the home page needs to highlight.