bug#39487: Broken links in the "Shepherd Services" section of the manual

[Alexandru-Sergiu Marton]

The "Shepherd Services" section of the GNU Guix manual contains a few
broken links that should point to the GNU Shepherd manual.

Search the page for the following text to find the broken links: Slots
of services, Service De- and Constructors, Invoking herd, Service
Convenience. Here is a comparison between a broken link and the
correct one:

broken: 
https://guix.gnu.org/manual/en/shepherd/Service-Convenience.html#Service-Convenience
correct: 
https://www.gnu.org/software/shepherd/manual/html_node/Service-Convenience.html#Service-Convenience

It seems that all the broken links follow the same pattern. To fix
them, it would be required to change the URL from
https://guix.gnu.org/manual/en/shepherd/<section_name>
to
https://www.gnu.org/software/shepherd/manual/html_node/<section_name>