Overlay component (#1998)

* Add styles, documentation, and storybook story for the new dialog component

* Stylelint auto-fixes

* Create slow-poets-mate.md

* Update slow-poets-mate.md

* update namespace

* Stylelint auto-fixes

* menu specifics

* starting over

* a new paradigm

* fully responsive

* push for testing

* auto width, visibility hidden

* Overlay stories

* dialog stories

* cleanup

* remove popover

* remove unused import

* docs update

* min() to the rescue!

* thin scrollbar?

* remove comment

* Update src/overlay/overlay.scss

Co-authored-by: simurai <simurai@github.com>

* address feedback

* handle form elements

* test permissions

* lint

Co-authored-by: Actions Auto Build <actions@github.com>
Co-authored-by: Katie Langerman <langermank@github.com>
Co-authored-by: simurai <simurai@github.com>

.changeset/slow-poets-mate.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": minor
+---
+
+Styles for the new Dialog Component 

docs/content/components/dialog.md

@@ -0,0 +1,58 @@
+---
+title: Dialog
+path: components/dialog
+status: Alpha
+source: 'https://github.com/github/github/tree/master/app/assets/stylesheets/experiments/modal-dialog.scss'
+bundle: dialog
+---
+
+Please note that the `<div>` element with `id="fake-container"` is not included in the component.
+
+```html live
+<div id="fake-container" style="height: 400px;">
+  <button class="btn"><span>Open dialog</span></button>
+  <div
+    id="overlay-backdrop"
+    class="Overlay-backdrop Overlay-backdrop--center"
+    role="dialog"
+    aria-labelledby="title-id"
+    aria-describedby="description-id"
+    data-focus-trap="active"
+  >
+    <div
+      class="Overlay Overlay--width-medium Overlay--height-medium Overlay--motion-scaleFade"
+      data-focus-trap="active"
+      open=""
+    >
+      <header class="Overlay-header">
+        <div class="Overlay-headerContentWrap">
+          <div class="Overlay-titleWrap">
+            <h1 id="title-id" class="Overlay-title">This is the title of the dialog</h1>
+            <h2 id="description-id" class="Overlay-description">This is the subtitle of the dialog</h2>
+          </div>
+          <div class="Overlay-actionWrap">
+            <button class="Overlay-closeButton" aria-label="Close">
+              <svg aria-hidden="true" viewBox="0 0 16 16" width="16" height="16" fill="currentColor">
+                <path
+                  fill-rule="evenodd"
+                  d="M3.72 3.72a.75.75 0 011.06 0L8 6.94l3.22-3.22a.75.75 0 111.06 1.06L9.06 8l3.22 3.22a.75.75 0 11-1.06 1.06L8 9.06l-3.22 3.22a.75.75 0 01-1.06-1.06L6.94 8 3.72 4.78a.75.75 0 010-1.06z"
+                ></path>
+              </svg>
+            </button>
+          </div>
+        </div>
+      </header>
+      <div class="Overlay-body">
+        This is the body of the dialogThis is the body of the dialogThis is the body of the dialog This is the body of
+        the dialog This is the body of the dialog This is the body of the dialog This is the body of the dialog This is
+        the body of the dialog This is the body of the dialog
+      </div>
+      <footer class="Overlay-footer Overlay-footer--divided Overlay-footer--alignEnd">
+        <button class="btn"><span>Continue</span></button>
+      </footer>
+    </div>
+  </div>
+</div>
+```
+
+[aria attributes]: https://www.w3.org/TR/html-aria/#allowed-aria-roles-states-and-properties