Static assets are re-fetched on every doc page load

There seems to be a problem with how the static assets on doc pages are cached in users' browsers. for example, loading this page: https://www.boost.org/doc/libs/latest/doc/antora/url/index.html appears to re-fetch a number of static assets each time, which can make the page load quite slowly.

Note: Google Chrome's `Disable cache` option was not selected for the test.