flip things around again, no more base class, try out some new stuff to keep responsive flex grid working

Should close https://github.com/twbs/bootstrap/issues/18556

Should close https://github.com/twbs/bootstrap/issues/18503

These can be replaced by their `.text-xs-*` parallels.
This also avoids any complications from interactions between the responsive and non-responsive classes.
(e.g. `<div class="text-left text-md-right">`)

Refs #18300
[skip sauce]

Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.

The actual em values were inconsistent with the docs,
while the docs were not the same as the comments:

* `sm` breakpoint was 34em (544px) not 480px.
* `lg` container max-width was 60rem (960px), less gutter than `md`.
  Changed to 940px, same as Bootstrap 3.
* `xl` container max-width was 72.25rem which is 1140px not 1156px.
  Changed to 1140px matching the comment but not the docs.

Addresses #17070 and #17388.

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

Fixes #17263.
[skip sauce]

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

Fix #17176

Sorry, I f'd up the rebase in #17186

Fix #17177

Simplest example is using horizontal variants using rows.

Mostly links.
Fixes #17056.

Fix: #17145

Fix #17129

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

- drop the index pages save for examples
- comment out the extend section for now
- fix up spacing fix on team list