Among other things, closes https://github.com/twbs/bootstrap/issues/26548

* update docs path from docs/4.0/ to docs/4.1/

* bump version to 4.1.0

* Update redirects to work for 4.1 docs move

* Update docs version switcher to include latest and link to 4.0 docs

* re-run dist

* Update package-lock.json

* Update docs-navbar.html

* Remove note about multiple inputs

* patchs the PR and removes the word Also

* Rewrite input group component

* Set the feedback to 100% width for input group

* Move from .row to .form-row for tighter layout

* no need for custom feedback here, we're using browser messaging

* add input group to validation examples

* add note about validating multiple

* migration note added

Closes #24437, closes #22457

While `placeholder` is nominally valid per spec
http://rawgit.com/w3c/html-api-map/master/index.html#accessible-name-and-description-calculation
it is inelegant, a fallback, and not supported in ios/safari/voiceover
and android/chrome/talkback, to name a few combinations

`aria-describedby` is also not really the right tool for providing a
name/label equivalent, so remove that from the list of possible
alternative methods / clarify how it can be used

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Fixes #20484

1. Invert `:first-child` into `:not(:last-child)` and vice versa
2. Remove double border at the left of `.form-control`
3. Shifts negative margins of `.btn` and `.btn-group` to retain
rightmost border when using at the left of `.form-control`.

[skip sauce]

[skip sauce]

- 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

[skip sauce]