x/website: cache-control

[Deleplace]

The static assets at go.dev are served without a cache-control response header, and are loaded from the server on every page view.

For example,

• https://go.dev/ is 65 requests, 241kB of compressed resources, every time I visit the page,
• https://go.dev/learn/ is 41 requests, 433kB of compressed resources, every time I visit the page.

I suggest we implement an HTTP caching strategy.

[cherrymui]

cc @golang/tools-team