Request to add link to docs on ngboost page

[ryan-wolbeck]

It's relatively hard to find the usage documentation from the main page of the project. My suggestion is to add a link to https://stanfordmlgroup.github.io/ngboost/intro.html next to the buttons below

[ryan-wolbeck]

@tonyduan

[tonyduan]

Thanks for ping @ryan-wolbeck, see above PR.