docs: Update Google Oauth set up instructions.
docs: Update Google Oauth set up instructions.

The flow seems to have changed a bit since these instructions were last
updated. Also information on which scopes needs to be authorized was
missing, which takes a bit of effort to figure and thus should be
written out explicitly.

kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip