x/website/content/static/doc/tutorial: needs to suggest 'go mod init' before 'go run .'

[hyangah]

https://tip.golang.org/doc/tutorial/getting-started#code

The first section needs update to include go mod init command.
Otherwise, users will face a compile error for their first program.

$ go run .
go: cannot find main module; see 'go help modules'

cc @stevetraut

[seankhliao]

https://tip.golang.org/doc/tutorial/getting-started#call

The second section also needs an explicit call to go get

» go run .
main.go:6:2: no required module provides package rsc.io/quote; to add it:
	go get rsc.io/quote

[gopherbot]

Change https://golang.org/cl/288277 mentions this issue: content: improve getting started document

[dmitshur]

This is the same as #43672, so I'll close this issue and update that one.