Improve page caching & add documentation.

- Disable page caching by default and enable it via the
`ENABLE_CACHING` env var.

-  Don’t cache no results page.
`Last Modified` is in code markup because it's the `Last-Modified` date in the request header? Is that right? You might want to include the hyphen and say "... `Last-Modified date in the request header to see if a new version ..."