Tests don't exist for testable Content Guide restrictions

[danswann]

Problem

Some parts of the Content Guide are testable, but no tests exist.
Examples:

• No links in headers
• No back-to-back links
• Linking to a given page at most once per section
• etc.

Context

Should add a new test action (that only runs on changes to /docs/*) which parses the .mdx files and tests for as much as possible to automate Content Guide compliance.
Probably better to set this up outside of the Docusaurus environment (e.g. with our own script that uses remark to parse the files) to avoid the tests running unnecessarily on every build.