doc: kresd.systemd belongs in section 7 of the manual

"man man" says that the sections are:

       1   Executable programs or shell commands
       2   System calls (functions provided by the kernel)
       3   Library calls (functions within program libraries)
       4   Special files (usually found in /dev)
       5   File formats and conventions eg /etc/passwd
       6   Games
       7   Miscellaneous  (including  macro  packages  and  conventions), e.g.
           man(7), groff(7)
       8   System administration commands (usually only for root)
       9   Kernel routines [Non standard]

Since there is no command named kresd.system it does not belong in
section 8.

Section 7 includes conventions and useful patterns like gitcli(7),
which seems more similar to the documentation that is supplied in
kresd.systemd.