Close#20976Close#20975
1. Fix the bug: the TOC in footer was incorrectly rendered as main
content's TOC
2. Fix the layout: on mobile, the TOC is put above the main content,
while the sidebar is put below the main content
3. Auto collapse the TOC on mobile
ps: many styles of "wiki.css" are moved from old css files, so leave
nits to following PRs.
