x/website: publish sitemaps

[hyangah]

go.dev has many pages. Publish sitemaps to help search engines and crawlers discover contents.

Related: #69600

Sitemaps: https://developers.google.com/search/docs/crawling-indexing/sitemaps/overview

go.dev endpoints: https://github.com/golang/website/blob/5f6954e6fcc9468bce96e01d8ebf374e37043a8c/cmd/golangorg/server.go#L153