Feature/1520 find related improvements

[ayebare]

Description of the Change

I have separated the find_related function into two. "get_related_query" which returns the query of get related posts, and "find_related" which uses "get_related_query",
No new logic was added.

Closes #1520

Benefits

Provide a function that returns WP_Query instance of related posts. This was requested from here #1520

Possible Drawbacks

None

Verification Process

wrote PHP unit tests for the new helper function and ran it

Checklist:

• [x ] I have read the CONTRIBUTING document.
• [x ] My code follows the code style of this project.
• [ x] My change requires a change to the documentation.
• [x ] I have updated the documentation accordingly.
• [ x] I have added tests to cover my change.
• All new and existing tests passed. [Some existing tests were failing before. All new tests pass.]

Applicable Issues

Changelog Entry