make TOC scrollable if higher than the viewport

[Pwuts]

Erratum on #1825

Problem:
If the sticky table of contents is higher than the viewport, the overflowing part can only be accessed by scrolling to the bottom of the page.

Solution:
Make the table of contents itself scrollable and limit its height to that of the viewport.

[Pwuts]

Screenshot shows a version with scrollbar, but imo the scrollbar was too prominent that way so I left it out in the PR.

Scrollbar additional code:

.page-col-sd {
  &::-webkit-scrollbar {
  /*   display: none; */
    width: 0.2em;
  }

  &::-webkit-scrollbar-track {
    margin: 12px;
    background-color: #e0e0e0;
  }

  &::-webkit-scrollbar-thumb {
    background-color: #1976d2;
  }
}

[Pwuts]

resolved conflict

[Pwuts]

@NGPixel I would advise processing this PR before the next release, as it should have been included in #1825 which has already been merged