Messaging linter does not check lists

[ptgott]

Let's say you have the following Markdown in a docs page:

---
title: Test Page
description: test page
---

This is a list:
1. This item has to do with the application service
2. This item has to do with the database service

This paragraph has to do with the kubernetes service.

The messaging linter—which checks whether the docs use Teleport terms correctly—will only catch "kubernetes service", not "application service" and "database service". We should extend the messaging linter to catch issues within lists (both ordered and unordered), and treat lists as part of the document body (you can configure the messaging linter to only check for errors in the title, description, headers, body, or code comments).