Adds documentation for the pronouns in the person schema

[vraja-pro]

Adds the pronouns to the person schema.

Fixes #https://github.com/Yoast/reserved-tasks/issues/808

Summary

Relevant technical choices

Technical choices that affect more than this issue:

Test instructions

This PR can be acceptance tested by following these steps:

1. Go to Yoast SEO Features -> Schema.org Markup -> Schema.org pieces -> Person, or /features/schema/pieces/person/
2. Go to Optional properties, /features/schema/pieces/person/#optional-properties.
3. Check you see the pronouns as part of the optional properties.

Quality assurance

• Security - I have thought about any security implications this code might add.
• Performance - I have checked that this code doesn't impact performance (greatly).
• Caching - I have analyzed the caching methods that this code touches and have added instructions to deal with those.
• Tested - I have tested this code to the best of my abilities.
• Automated tests - I have added unit tests to verify the code works as intended.
• Testability - I have added unique ids to elements, so they can be located in automated testing.
• I have altered a filename.
  • I have adjusted the ID property accordingly and updated all internal links.
  • I have added the redirect to the _redirects file in the root of the project.

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