Building a Treatments Page, What Do I Use?

As @N00b pointed out this can go in many answers and needs many concepts digestion.I will try to concise and give a sample answer with little info.

Since you have set has_archive to true while registering post type , we can see all treatment post-type post on example.com/treatment/. If we want to change this page to display just taxonomy categories list, we can use template page for this.

Create archive-treatment.php in the root directory of the theme (If you know how to code and create child themes or the theme is your own ). Example archive-treament.php looks like.

archive-treatment.php

<?php
    get_header();
    $args = array(
        'show_option_none'    => __( 'No treatment categories' ),
        'style'               => 'list',
        'taxonomy'            => 'treatment_category',
        'title_li'            => __( 'Treatment Categories' )
    );
    wp_list_categories($args);
    get_footer();

Here wp_list_categories outputs the categories available in the treatment_category taxonomy and check all the arguments available.

If you just want to create page and use shortcodes there are many plugins out there that do this.

Reference:

Template Hierarchy

wp_list_categories

Display Posts Shortcode