Improve wiki sidebar and TOC (#25460)

Close #20976
Close #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.

web_src/js/modules/fomantic.js

@@ -3,6 +3,8 @@ import {initAriaCheckboxPatch} from './aria/checkbox.js';
 import {initAriaDropdownPatch} from './aria/dropdown.js';
 import {svg} from '../svg.js';
 
+export const fomanticMobileScreen = window.matchMedia('only screen and (max-width: 767.98px)');
+
 export function initGiteaFomantic() {
   // Silence fomantic's error logging when tabs are used without a target content element
   $.fn.tab.settings.silent = true;