docs: new and improve lexical docs, hoist up all headings (#6639)

docs/admin/customizing-css.mdx

@@ -6,7 +6,7 @@ desc: Customize your Payload admin panel further by adding your own CSS or SCSS
 keywords: admin, css, scss, documentation, Content Management System, cms, headless, javascript, node, react, express
 ---
 
-### Adding your own CSS / SCSS
+## Adding your own CSS / SCSS
 
 You can add your own CSS by providing your base Payload config with a path to your own CSS or SCSS. Customize the styling of any part of the Payload dashboard as necessary.
 
@@ -25,7 +25,7 @@ const config = buildConfig({
 })
 ```
 
-### Overriding built-in styles
+## Overriding built-in styles
 
 To make it as easy as possible for you to override our styles, Payload uses [BEM naming conventions](http://getbem.com/) for all CSS within the Admin UI. If you provide your own CSS, you can override any built-in styles easily.
 
@@ -41,7 +41,7 @@ You can find the built-in Payload CSS variables within [`./src/admin/scss/app.sc
 - Fonts
 - Horizontal gutter
 
-#### Dark mode
+### Dark mode
 
 <Banner type="warning">
   If you're overriding colors or theme elevations, make sure to consider how your changes will