fix(docs-infra): fix layout of `file-not-found` page (#31390)

The `.sidenav-content` element, [which is also][1] the `#file-not-found`
element, [already has][2] the necessary padding. Especially the top
padding is important to ensure the element is not obscured by the
top-bar.

[1]: https://github.com/angular/angular/blob/1d3e22766/aio/src/app/app.component.html#L51
[2]: https://github.com/angular/angular/blob/1d3e22766/aio/src/styles/1-layouts/_content-layout.scss#L10

PR Close #31390

aio/src/styles/1-layouts/_not-found.scss

@@ -1,7 +1,3 @@
-#file-not-found {
-    padding: 3rem 3rem 3rem;
-}
-
 .nf-container {
     align-items: center;
     padding: 32px;