* Expand advice for anchor-based controls

* Expand accessibility note in input group

* Correct statement about validation, fix server example

* Tweak label > accessible name

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Mark Otto <markd.otto@gmail.com>

* Prep for v4.3.0

* More updates for v4.3.0.

* rerun to build docs source map

- Move 4.1 docs to 4.2
- Update versions everywhere to 4.1.3 with release script
- Manually bump the shorthand version in package.json
- Add 4.2 to the versions docs page
- Update some redirects
- Fix tests asset URLs
- Bump Nuget and more

Fixes #25409

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

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