doc.rust-lang.org: add caching headers

(moving from https://github.com/rust-lang/rust/issues/82286)

Steps to reproduce:

1. Load any doc.rust-lang.org page, e.g. https://doc.rust-lang.org/std/str/index.html.
2. Open Developer Console.
3. Click the URL bar and hit enter to load the page again.

Expected result:

Most resources are cached and do not hit the network.

Actual result:

Many objects require a network fetch, but get a 304 Not Modified. These should be served with long caching headers.

Current headers:

$ curl -I https://doc.rust-lang.org/SourceCodePro-Regular.woff
HTTP/2 200 
content-type: font/woff
content-length: 55472
last-modified: Thu, 11 Feb 2021 14:27:46 GMT
x-amz-version-id: R.xOjlR7eKEsQwzm9EIrHs9eeCIDeXYq
server: AmazonS3
date: Thu, 18 Feb 2021 13:27:31 GMT
etag: "957fa8c030f8116bea59c13df470e4e8"
x-cache: Hit from cloudfront
via: 1.1 4b84530d7a095b58fb7a1d20b7f0cbe0.cloudfront.net (CloudFront)
x-amz-cf-pop: HIO50-C2
x-amz-cf-id: QpMYsW6BVcTPpDmi8Pg9HiMz_1iSOIpdYv60MMzNqVoV707yqsv_Dw==
age: 63023

Reproduce the issue with the documented doc.rust-lang.org URL and the shown curl command. The payload names no repository files or entry points, so first locate the Amazon S3 or CloudFront configuration in simpleinfra, then verify that static resources receive long caching headers and reload without unnecessary 304 network requests.