Create a basic to-dos Vale test

We should use "to-do items" instead of "to dos" - this test needs to
be made more comprehensive, but this is an MVC.

doc/.vale/gitlab/ToDo.yml

+---
+# Warning: gitlab.ToDo
+#
+# You should not use "To Do", unless it refers to the UI element.
+#
+# For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
+extends: substitution
+message: 'Use "%s" instead of "%s" unless referring to the "Add a To Do" button in the UI.'
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#feature-names
+level: warning
+ignorecase: true
+swap:
+  'to dos': to-do items

doc/development/documentation/styleguide.md

@@ -369,7 +369,7 @@ create an issue or an MR to propose a change to the user interface text.
   - milestones
   - reorder issues
   - runner, runners, shared runners
-  - a to-do item, to dos
+  - a to-do item (tested in [`ToDo.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/ToDo.yml))
 - *Some features are capitalized*, typically nouns naming GitLab-specific
   capabilities or tools. For example:
   - GitLab CI/CD