bug#57046: Spanish documentation uses exclusive language

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

Cc to the Spanish translators in po/doc/guix-manual.es.po

Thank you Jean Pierre De Jesus DIAZ for filing a bug instead of starting
an edit war on Weblate.  But if there is a decision on changing, someone
will have to edit the translation on Weblate.

For comparison,

* the main Spanish translation po/guix/es.po uses usuario

* the French translation switches between “utilisateur·rices”,
  “utilisatrices et utilisateurs” and more often masculine “utilisateurs”

* the Portuguese, Russian, German translation use masculine (although at
  least for German I use feminine in some examples)

* German word for users is masculine Benutzer and feminine is
  Benutzerinnen; there is a psychology study that Benutzer*innen is
  perceived feminine while listing both masculine and feminine Benutzer
  und Benutzerinnen is perceived as including men and women (a different
  language and I might give too much weight to one scientific study)
  <https://www.hw.uni-wuerzburg.de/aktuelles/news/single/news/gendersternchen-lassen-an-frauen-denken/>

Regards,
Florian