fix(docs-infra): prevent main content from overlapping with ToC (#42884)

Previously, if there was more content than what would horizontally fit
in the main content area, it would overflow to the right and overlap
with the Table of Contents (ToC).
This was accidentally introduced in #42787.

This commit fixes it by ensuring that the main content area will not
overlap with ToC and the necessary space for the ToC will be reserved
using `margin` (instead of `padding`, which contributes to the element's
size).

Fixes #42867

PR Close #42884

aio/src/styles/1-layouts/sidenav/_sidenav.scss

@@ -14,7 +14,7 @@ mat-sidenav-container.sidenav-container {
 
   &.has-floating-toc {
     .mat-sidenav-content {
-      padding-right: 18vw;
+      margin-right: 18vw;
     }
   }