fix #58023

docs for adding Saturday for first day of the week
add related settings for Saturday as first day of the week
firstDayOfWeek: Use enumeration, replace day's numbers with corresponding names
make some variables lowercase (follow camelCase)
add CHANGELOG entry

Author:    haghighi_ahmad <haghighi.ahmad@gmail.com>

modified:   app/assets/javascripts/pages/users/activity_calendar.js
modified:   app/helpers/preferences_helper.rb
new file:   changelogs/unreleased/58023-add-Saturday-to-localization-first-day-of-the-week.yml
modified:   doc/api/settings.md
modified:   doc/user/profile/preferences.md
modified:   locale/gitlab.pot
modified:   spec/helpers/preferences_helper_spec.rb