services: cgit: Add support for project-list.

* doc/guix.texi (Version Control Services): Update accordingly.
* gnu/services/cgit.scm (cgit-configuration)[project-list]: New field.
(serialize-project-list): New procedure that uses PLAIN-FILE to generate a
file from the string list given by the user as input.
(serialize-cgit-configuration): Make sure to serialize 'project-list' before
'repostory-directory'.