Hi Steve
In the front page of your site, add a Main Menu block. Add a page to that called About Us. Add the content.
If you want to list the About Us to a menu, note the url of the page and use the theme settings to add it to a menu. You can optionally hide the Main Menu block (not the Page!!) if you want to make things tidy.
Worked example -
https://www.smmc.org.uk/mod/page/view.php?id=6
Docs -
https://docs.moodle.org/39/en/Front_page_FAQ#How_can_I_add_a_page_to_the_front_page.3F
https://docs.moodle.org/39/en/Theme_settings#Custom_menu_items