/cc #23070

Bootstrap itself doesn't use any `::first-letter` or `::first-line`
styles. These selectors also cause(d) problems in IE11 (see
https://github.com/h5bp/html5-boilerplate/pull/1799) and currently cause
a nasty rendering bug in Chrome where the first letter is vertically
shifted (see https://github.com/twbs/bootstrap/issues/21771)

As h5bp has now removed these as well, no need to keep and comment them
out. See https://github.com/h5bp/html5-boilerplate/issues/1961 /
https://github.com/h5bp/html5-boilerplate/pull/1962

move the about directory into the docs to avoid collision with the multiple versions of our hosted docs

When focused, these buttons styled as links still give good visible focus indication through the underline, so this should not be a concern. Closes #22655

also remove the overflow-x. when you specify either overflow-y or overflow-x, the other is essentially nullified and thus ineffective anyway

fixes #22977

The Japanese website (http://bootstrap4.jp/) doesn't exist therefore I removed the link to it.