Fix anchors, fix naming consistency

Replace remaining instances of defunct .visible* classes in docs

.visible-xs(-block) => .hidden-sm-up

Fixes #17263.
[skip sauce]

Fix accidental triple colon from :before to ::before conversion

Fix #17288 — Sorry about that!

Use double colons for psuedo elements

Fix #17052

I found non working anchors and some naming in consistency. I hope I fixed well. Have a nice day.

Add jekyll-redirect-from plugin in Gemfile

/cc @cvrebert

v4 - Document migration from panels to cards

v4 - Dropup caret direction incorrect

New URL for Chinese translation of Bootstrap 4.x

http://v4.bootcss.com

Add jekyll-redirect-from plugin in Gemfile, ohterwise report ".../jekyll/plugin_mannager.rb:29:in require: cannot load such file -- jekyll-redirect-from (LoadError)" when jekyll build

Apply !default to SCSS variables

!default to all variables.

Add white background to cards by default

Fix #17144

Fix #17117

Fixes grammatical erratum from cf7819d8