docs: Customize view transition and other layout shift improvements (#43763)

Follow up to https://github.com/zed-industries/zed/pull/43762.

Yet another shot at making the navigation between pages flicker less.

Release Notes:

- N/A

docs/theme/page-toc.js

@@ -56,7 +56,7 @@ const updateFunction = () => {
   }
 };
 
-window.addEventListener("load", () => {
+document.addEventListener("DOMContentLoaded", () => {
   const pagetoc = getPagetoc();
   const headers = [...document.getElementsByClassName("header")];