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-workers-and-pages]

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