docs(google-maps): Improve documentation for Google Maps.

[mbehrlich]

Add new Google Maps components to documentation, including MapPolygon,
MapPolyline, MapRectangle, MapCircle, MapGroundOverlay, MapKmlLayer,
MapTransitLayer, MapTrafficLayer, and MapBicyclingLayer. Add
instructions on how to lazy load the Google Maps JavaScript API for use
with the @angular/google-maps components.

[jelbourn]

LGTM, just a couple tiny capitalization nits

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[weilinzung]

@mbehrlich @jelbourn

The lazyload example has an issue with You have included the Google Maps JavaScript API multiple times on this page. This may cause unexpected errors. when the same Google map component is used multiple times within a single-page app.

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