fix(docs-infra): fix styling of embedded Table-of-Contents (#34312)

This commit fixes the following issues:
- Align text content with caret icon in embedded ToC header.
- Fix display of expand/collapse button at the bottom of embeded ToC.

These were accidentally broken in #32124, while fixing another
ToC-related issue. Some CSS rules in `_toc.scss` (e.g. those for
`.mat-icon` and `ul.toc-list`) were correctly moved out of the
`.toc-inner > button` block, but others (e.g. `&.toc-heading` and
`&.toc-more-items`) should not have been moved outside the
`.toc-inner > button` block.

_Before:_
![toc-styles before](https://user-images.githubusercontent.com/8604205/70441952-00df2380-1a9e-11ea-8fc7-d141cc8c9045.png)

_After:_
![toc-styles after](https://user-images.githubusercontent.com/8604205/70441953-0177ba00-1a9e-11ea-8af8-d497977009ae.png)

Closes #34300

PR Close #34312

aio/src/styles/2-modules/_toc.scss

@@ -50,7 +50,6 @@
         background: $lightgray;
       }
     }
-  }
 
     &.toc-heading {
       mat-icon.rotating-icon {
@@ -83,6 +82,7 @@
         content: 'more_horiz';
       }
     }
+  }
 
   .mat-icon {
     &.collapsed {