Docs: try out suggestion-level substitutions

Our current substitutions file in Vale is set to the error level,
which is not appropriate for all of the suggestions we want to make
to the technical writing team. This commit adds another substition
file with a name that makes clear the items within it are at a
suggestion level, rather than an error level.

doc/.vale/gitlab/AfterOnce.yml

+---
+# Suggestion: gitlab.AfterOnce
+#
+# 'Once' is frequently misused at GitLab when the user means 'after'.
+#
+# For a list of all options, see https://errata-ai.github.io/vale/styles/
+extends: substitution
+message: 'Usage check: Use "once" for counting ("Run a job once...") and "after" for a sequence ("Run this job after...").'
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#language
+level: suggestion
+ignorecase: true
+swap:
+  once: after

doc/.vale/gitlab/SubstitutionSuggestions.yml

+---
+# Suggestion: gitlab.SubstitutionSuggestions
+#
+# Suggests better options for frequently misused terms at GitLab that are
+# often - but not always - incorrect.
+#
+# For a list of all options, see https://errata-ai.github.io/vale/styles/
+extends: substitution
+message: 'Consider "%s" instead of "%s".'
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#language
+level: suggestion
+ignorecase: true
+swap:
+  since: because
+  utilize: use