* gnu/services/web.scm (<nginx-configuration>)[server-blocks]: New
field.
(nginx-activation): When CONFIG-FILE is #f, use 'default-nginx-config'.
(nginx-shepherd-service): Likewise.
(nginx-service-type): Add 'compose' and 'extend' fields.
(nginx-service): Change default value of #:server-list to '(), and
default value of #:config-file to #f.
* doc/guix.texi (Web Services): Document it.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>