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
