Add Vale as an English language linter

Vale (https://errata-ai.github.io/vale/) is a grammar, style, and word
usage linter for the English language. This merge request adds an
INI file for Vale (.vale.ini) to the root directory of the repository,
and creates a hidden folder (doc/.linting) for the storage of linting
tests.

Since the TW team uses multiple linting tools, and Vale also supports
multiple collections of rules (called 'styles'), the path to the tests
folder is doc/.linting/vale/styles/gitlab to support any future
expansions that can't be predicted at this time.

The folder currently contains only a .gitkeep file to meet the goal
of a MVC. Tests should be developed, vetted, and added in separate
merge requests, but those MRs all depend on the addition of the
INI file and the directory structure.

.vale.ini

+# Vale configuration file, taken from https://errata-ai.github.io/vale/config/
+
+# The relative path to the folder containing linting rules (styles)
+# -----------------------------------------------------------------
+StylesPath = doc/.linting/vale/styles
+
+# Minimum alert level
+# -------------------
+# The minimum alert level to display (suggestion, warning, or error).
+# If integrated into CI, builds fail by default on error-level alerts,
+# unless you execute Vale with the --no-exit flag
+MinAlertLevel = suggestion
+
+# Should Vale parse any file formats other than .md files as Markdown?
+# --------------------------------------------------------------------
+[formats]
+mdx = md
+
+# What file types should Vale test?
+# ----------------------------------
+[*.md]
+
+# Styles to load
+# --------------
+# What styles, located in the StylesPath folder, should Vale load?
+# Vale also currently includes write-good, proselint, joblint, and vale
+BasedOnStyles = gitlab
+
+# Enabling or disabling specific rules in a style
+# -----------------------------------------------
+# To disable a rule in an enabled style, use the following format:
+# {style}.{filename} = NO
+# To enable a single rule in a disabled style, use the following format:
+# vale.Editorializing = YES
+
+# Altering the severity of a rule in a style
+# ------------------------------------------
+# To change the reporting level (suggestion, warning, error) of a rule,
+# use the following format: {style}.{filename} = {level}
+# vale.Hedging = error
+
+# Syntax-specific settings
+# ------------------------
+# You can configure specific tests to be enabled, disabled, or report at a 
+# different level for specific file types. File-type-specific settings added
+# here will overwrite any conflicting global settings.
+[*.{md,txt}]
+# vale.Editorializing = NO
+

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

+extends: existence
+message: Use a comma before the last "and" in a list of three or more items.
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#punctuation
+level: warning
+tokens:
+  - '(?:[^,]+,){1,}\s\w+\sand'

doc/development/documentation/index.md

@@ -420,6 +420,7 @@ The following are some suggested linters you can install locally and sample conf
 
 - [`proselint`](#proselint)
 - [markdownlint](#markdownlint), which is the same as the test run in [`docs-lint`](#testing)
+- [Vale](#vale), for English language grammar and syntax suggestions
 
 NOTE: **Note:**
 This list does not limit what other linters you can add to your local documentation writing toolchain.
@@ -519,6 +520,23 @@ also check [the issue](https://gitlab.com/gitlab-org/gitlab-foss/issues/64352) t
 tracked the changes required to implement these rules, and details which rules were
 on or off when markdownlint was enabled on the docs.
 
+#### `Vale`
+
+[Vale](https://errata-ai.github.io/vale/) is a grammar, style, and word usage linter
+for the English language. Vale's configuration is stored in the
+[`.vale.ini`](https://gitlab.com/gitlab-org/gitlab/blob/master/.vale.ini) file
+located in the root directory of the [GitLab repository](https://gitlab.com/gitlab-org/gitlab).
+
+Vale supports creating [custom tests](https://errata-ai.github.io/vale/styles/),
+stored in the `doc/.linting/vale/styles/gitlab` directory, that extend any of
+several types of checks.
+
+To view linting suggestions locally, you must install Vale on your own machine, and
+[configure the text editor of your choice](https://errata-ai.github.io/vale/#local-use-by-a-single-writer)
+to display the results.
+
+Vale's test results are not currently displayed in CI, but may be displayed in the future.
+
 ## Danger Bot
 
 GitLab uses [Danger](https://github.com/danger/danger) for some elements in