Create substitution word test for Vale

GitLab's style guide provides guidance for some word pairs, but
the technical writing team has no way of being alerted if those
word pairs are in use. This commit creates the beginning of a word
substitution test.

doc/.linting/vale/styles/gitlab/Substitutions.yml

+---
+# `extends` indicates the Vale extension point being used.
+# Full list of styles: https://errata-ai.github.io/vale/styles/
+extends: substitution
+
+# Substitution rules can display the matched and suggested strings in the 
+# message shown to the user. The first use of %s prints the suggested option,
+# and the second use of %s displays what was found in the text.
+message: Use "%s" instead of "%s."
+
+# Should a result be flagged as a suggestion, warning, or error?
+# Results that fall below the MinAlertLevel set in 
+# https://gitlab.com/gitlab-org/gitlab/blob/master/.vale.ini won't be shown.
+level: warning
+
+# Should a match be case-insensitive or case-sensitive?
+# Acceptable values are 'true' or 'false'
+ignorecase: true
+
+# Should this rule be limited to a specific scope? If yes, uncomment the line.
+# Possible scopes: https://errata-ai.github.io/vale/formats/#available-scopes
+# scope: heading
+
+# Should this rule ignore normal word boundaries, such as \b ?
+# Acceptable values are 'true' or 'false'
+nonword: true
+
+# What is the source for this rule?
+link: https://about.gitlab.com/handbook/communication/#top-misused-terms
+
+# The 'swap' section provides a list of values, one per line, in the form of
+# $bad: $good
+swap:
+  GitLabber: GitLab team member
+  self hosted: self-managed
+  self-hosted: self-managed