Table of contents should be sticky

[Thomas Luzat]

When scrolling down with visible table of contents (larger viewport sizes) on pages such as

http://manpages.ubuntu.com/manpages/hirsute/en/man1/ssh-keygen.1.html

the table of contents moves out of view which complicates navigation. I propose adding CSS similar to the following:

#toc {
    position: sticky;
    top: 0;
    max-height: 100vh;
    overflow: auto;
}

This makes the ToC stick in position for current browsers. Additionally, large ToCs become scrollable, which may be required if the TOC doesn't fit the screen, like on

http://manpages.ubuntu.com/manpages/hirsute/en/man1/rsync.1.html

For old browsers not supporting some of the properties, this should break pretty gracefully.

A nice addition would be to highlight the currently visible section(s), but this would be a separate feature and probably require a bit of JS.