Add documentation for wpseo_sitemap_public_url filter

[pls78]

Fixes #323

Summary

This PR added documentation for the new wpseo_sitemap_public_url filter.

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.

[cloudflare-pages[bot]]

Deploying yoast-developer with    Cloudflare Pages

Latest commit:	49ba8e7
Status:	✅  Deploy successful!
Preview URL:	https://8a75d035.yoast-developer.pages.dev
Branch Preview URL:	https://add-wpseo-sitemap-public-url.yoast-developer.pages.dev

View logs