[docs] Ubuntu fastcgi_cache_purge package

[stashymane]

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 (and related) should be updated to encourage installing the module instead :)

[matrixkloud]

this is recommended. even cache purge through nginx helper is not removing cache file due to different nginx user and fpm process user.
if nginx can run under user www-data then clear cache would be easy. otherwise, everytime, need to ssh into server and run the clear cache command for proxy cache.