v5: Forms update (#28450)

* Initial spike of consolidated form checks

* Stub out forms rearrangement

- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS

* Move input group Sass file to forms subdir

* Start to split and move the docs around

* Simpler imports

* Copyediting

* delete overview file

* Remove commented out code

* remove the custom-forms import

* rewrite flex-check as form-check, replace all custom properties

* Remove old forms doc

* stub out new subpage link section

* update migration guide

* Update nav, forms overview in page nav, and descriptions

* fix check bg position

* fix margin-top calculation

* rename .custom-select to .form-select

* Update validation styles for new checks

* add some vertical margin, fix inline checks

* fix docs examples

* better way to do this contents stuff, redo the toc while i'm at it

* page restyle for docs while here

* un-callout that, edit text

* redo padding on toc

* fix toc

* start to cleanup checks docs

* Rewrite Markdown tables into HTML

* Redesign tables, redo their docs

* Replace Open Iconic icons with custom Bootstrap icons

* Redesign the docs navbar, add a subheader, redo the sidebar

* Redesign docs homepage a bit

* Simplify table style overrides for docs tables

* Simplify docs typography for page titles and reading line length

* Stub out icons page

* Part of sidebar update, remove migration from nav.yml

* Move toc CSS to separate partial

* Change appearance of overview page

* fix sidebar arrow direction

* Add footer to docs layout

* Update descriptions

* Drop the .form-group class for margin utilities

* Remove lingering form-group-margin-bottom var

* improve footer spacing

* add headings to range page

* uncomment form range css

* Rename .custom-range to .form-range

* Drop unused docs var

* Uncomment the comment

* Remove unused variable

* Fix radio image sizing

* Reboot update: reset horizontal ul and ol padding

* de-dupe IDs

* tweak toc styles

* nvm, fix dropdown versions stuff

* remove sidebar nav toggle for now

* broken html

* fix more broken html, move css

* scss linting

* comment out broken helper docs

* scope styles

* scope styles

* Fixes #25540 and fixes #26407 for v5 only

* Update sidebar once more

* Match new sidenav order

* fix syntax error

* Rename custom-file to form-file, update paths, update migration docs for previous changes in #28696

* rename back

* fix size and alignment

* rename that back too

site/content/docs/4.3/forms/form-control.md

@@ -0,0 +1,75 @@
+---
+layout: docs
+title: Form controls
+description: Give textual form controls like `<input>`s and `<textarea>`s an upgrade with custom styles, sizing, focus states, and more.
+group: forms
+toc: true
+---
+
+## Example
+
+{{< example >}}
+<form>
+  <div class="mb-3">
+    <label for="exampleFormControlInput1">Email address</label>
+    <input type="email" class="form-control" id="exampleFormControlInput1" placeholder="name@example.com">
+  </div>
+  <div class="mb-3">
+    <label for="exampleFormControlTextarea1">Example textarea</label>
+    <textarea class="form-control" id="exampleFormControlTextarea1" rows="3"></textarea>
+  </div>
+</form>
+{{< /example >}}
+
+## Sizing
+
+Set heights using classes like `.form-control-lg` and `.form-control-sm`.
+
+{{< example >}}
+<input class="form-control form-control-lg" type="text" placeholder=".form-control-lg">
+<input class="form-control" type="text" placeholder="Default input">
+<input class="form-control form-control-sm" type="text" placeholder=".form-control-sm">
+{{< /example >}}
+
+## Readonly
+
+Add the `readonly` boolean attribute on an input to prevent modification of the input's value. Read-only inputs appear lighter (just like disabled inputs), but retain the standard cursor.
+
+{{< example >}}
+<input class="form-control" type="text" placeholder="Readonly input here..." readonly>
+{{< /example >}}
+
+## Readonly plain text
+
+If you want to have `<input readonly>` elements in your form styled as plain text, use the `.form-control-plaintext` class to remove the default form field styling and preserve the correct margin and padding.
+
+{{< example >}}
+<form>
+  <div class="mb-3 row">
+    <label for="staticEmail" class="col-sm-2 col-form-label">Email</label>
+    <div class="col-sm-10">
+      <input type="text" readonly class="form-control-plaintext" id="staticEmail" value="email@example.com">
+    </div>
+  </div>
+  <div class="mb-3 row">
+    <label for="inputPassword" class="col-sm-2 col-form-label">Password</label>
+    <div class="col-sm-10">
+      <input type="password" class="form-control" id="inputPassword">
+    </div>
+  </div>
+</form>
+{{< /example >}}
+
+{{< example >}}
+<form class="form-inline">
+  <div class="mb-3 mb-2">
+    <label for="staticEmail2" class="sr-only">Email</label>
+    <input type="text" readonly class="form-control-plaintext" id="staticEmail2" value="email@example.com">
+  </div>
+  <div class="mb-3 mx-sm-3 mb-2">
+    <label for="inputPassword2" class="sr-only">Password</label>
+    <input type="password" class="form-control" id="inputPassword2" placeholder="Password">
+  </div>
+  <button type="submit" class="btn btn-primary mb-2">Confirm identity</button>
+</form>
+{{< /example >}}