refactor(docs-infra): avoid Material style duplication warning (#37898)

This commit removes some duplicate imports of Material themes and
theming-related utilities. While this change does not have any impact on
the size of the generated `styles.css` file, it silences a build warning
pointing to [Avoiding duplicated theming styles][1].

[1]: https://github.com/angular/components/blob/db4b0cd1bff0bbb1741d22975bcfe1069aea3a53/guides/duplicate-theming-styles.md

PR Close #37898

aio/angular.json

@@ -58,7 +58,7 @@
               }
             ],
             "styles": [
-              "src/styles.scss"
+              "src/styles/main.scss"
             ],
             "scripts": [],
             "budgets": [
@@ -158,7 +158,7 @@
               }
             ],
             "styles": [
-              "src/styles.scss"
+              "src/styles/main.scss"
             ],
             "scripts": []
           }
@@ -193,4 +193,4 @@
     }
   },
   "defaultProject": "site"
-}
+}

aio/src/styles.scss

@@ -1,3 +0,0 @@
-/* You can add global styles to this file, and also import other style files */
-@import './styles/ng-io-theme';
-@import './styles/main.scss';

aio/src/styles/main.scss

@@ -1,5 +1,4 @@
 // import global themes
-@import '~@angular/material/theming';
 @import './ng-io-theme';
 
 // import global variables