Update custom theme docs and path checks for custom templates

[cristianuibar]

custom.template.php is being removed when a plugin update is installed. We should no longer recommend this method of customization for the template files. Instead, we should recommend using a child theme and installing it in the main folder of this child theme instead.

The correct path would be: wp-content/themes/<child-theme-slug>/toplytics-template.php

Original report here: https://wordpress.org/support/topic/custom-template-php-deleted-while-updating/