Docs: initial test for future tense

The GitLab style guide discourages the use of future tense. This MR
attempts to capture some of the most common word variants for future
tense.

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

+---
+# Warning: gitlab.FutureTense
+#
+# Checks for use of future tense in sentences. Present tense is preferred as
+# much as possible.
+#
+# For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
+extends: existence
+message: 'Avoid using future tense: "%s"'
+ignorecase: true
+level: suggestion
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#language-to-avoid
+tokens:
+  - going to be
+  - going to
+  - he'll
+  - she'll
+  - they'll
+  - we'll
+  - will be
+  - will have
+  - will # Leave this word after the two-word 'will' variants as a catchall
+  - won't
+  - you'll