fix(docs-infra): fix the layout of docs pages in IE11 (#41183)

IE11 does not recognize the `<main>` element as a block element and
therefore it does not apply margin/paddings/widths as expected. This
resulted in the layout of docs pages being broken in IE11 (see the image
below).

This commit fixes the layout by explicitly setting `.sidenav-content`
(which is a `<main>` element) to `display: block`.

Before: ![docs pages layout in IE11 before][1]
After: ![docs pages layout in IE11 after][2]

[1]: https://user-images.githubusercontent.com/8604205/110808902-1b014600-828d-11eb-9eef-c1234dc2d436.png
[2]: https://user-images.githubusercontent.com/8604205/110808907-1ccb0980-828d-11eb-91d9-06e5cfafc894.png

PR Close #41183

aio/src/styles/1-layouts/_content-layout.scss

@@ -4,6 +4,8 @@
 }
 
 .sidenav-content {
+  display: block;  // This is required for browsers that do not recognize `<main>` as a block
+                   // element (such as IE11).
   min-height: 100vh;
   padding: 80px 3rem 2rem;
   max-width: 50em;