[docs] Ubuntu fastcgi_cache_purge package

Instead of recompiling nginx with fastcgi_cache_purge or installing a custom build of it, there is a `libnginx-mod-http-cache-purge` package available in the universe repo, completely hassle-free - https://www.ubuntuupdates.org/package/core/lunar/universe/base/libnginx-mod-http-cache-purge
[This guide](https://easyengine.io/wordpress-nginx/tutorials/single-site/fastcgi-cache-with-purging/) (and related) should be updated to encourage installing the module instead :)