firestore: add imports to godoc
firestore: add imports to godoc

Add imports at the top of site page

Fixes #1940

Change-Id: I579b113add5c3c764edcedba2836bef7d1e3d8e9
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/56330
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go