Allow users to filter the number of results per page in Instant Results

[JakePT]

Description of the Change

This PR does two things:

1. Sets the default number of results per page in Instant Results to match the number of posts per page as set in Settings > Reading. This is configured as the default value for a feature setting so that a UI could be supported in future.
2. Adds an ep_instant_results_per_page filter so developers can change the number of results per page in Instant Results.

Also included are a test for both of the above pieces of functionality and a section in the theme integration docs on using the filter.

Closes #3274

How to test the Change

1. Activate Instant Results and perform a search. The number of results should match the number of posts per page as set in Settings > Reading.
2. Add code like add_filter( 'ep_instant_results_per_page', function() { return 3; } ); and verify that the number of results per page matches the filtered value.

Changelog Entry

Added - An ep_instant_results_per_page filter for changing the number of results per page in Instant Results.
Changed - The number of results per page in Instant Results now matches the site's posts per page setting.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

@JakePT we have an e2e test that is consistently failing on this one. Did you have the chance to give a look into it?