This helpful when adding content to the nginx configuration file, which isn't
supported by the record type used for the configuration. For example, like
adding proxy_cache_path configuration.
* gnu/packages/web.scm (<nginx-configuration>): Add new extra-content field.
(nginx-configuration-extra-content): New field accessor.
(default-nginx-config): Add support for the extra-content field.
* doc/guix.texi (NGINX): Document the new extra-content field.
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