Update the cleanup policy docs

What does this MR do?

Update the order of operations in How the cleanup policy works documentation

According to https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/projects/container_repository/cleanup_tags_service.rb#L60 both name_regex and name_regex_keep are evaluated at once which produces a different result than expected from the docs which can confuse customers trying to understand the cleanup policy (as learned in internal ZD ticket

If we'd want to retain the currently documented order of operations we would need to split

      def filter_by_name(tags)
        regex_delete = ::Gitlab::UntrustedRegexp.new("\\A#{params['name_regex_delete'] || params['name_regex']}\\z")
        regex_retain = ::Gitlab::UntrustedRegexp.new("\\A#{params['name_regex_keep']}\\z")

        tags.select do |tag|
          # regex_retain will override any overlapping matches by regex_delete
          regex_delete.match?(tag.name) && !regex_retain.match?(tag.name)
        end
      end

into 2 steps, one doing regex_delete just where it is and the other one doing regex_retain after https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/projects/container_repository/cleanup_tags_service.rb#L21

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