docs(popover): new component usage and style tab content

src/data/nav-items.yaml

@@ -146,6 +146,8 @@
       path: /components/overflow-menu/usage/
     - title: Pagination
       path: /components/pagination/usage/
+    - title: Popover
+      path: /components/popover/usage/
     - title: Progress indicator
       path: /components/progress-indicator/usage/
     - title: Radio button

src/pages/components/popover/style.mdx

@@ -0,0 +1,56 @@
+---
+title: Popover
+description:
+  A popover is a layer that pops up over all other elements on a page.
+tabs: ['Usage', 'Style']
+---
+
+## Color
+
+| Element   | Property         | Color token           |
+| --------- | ---------------- | --------------------- |
+| Container | background-color | `$layer-01`           |
+|           | background-color | `$layer-02`           |
+|           | background-color | `$background-inverse` |
+
+<Row>
+<Column colLg={8}>
+
+![Popover container colors.](images/popover-style-1.png)
+
+</Column>
+</Row>
+
+## Structure
+
+The width and height of a popover container can vary depending on the amount of
+content within it. We recommend to not exceed a popover width size of four
+columns.
+
+| Element   | Property      | px / rem | Spacing token |
+| --------- | ------------- | -------- | ------------- |
+| Container | max-width     | 352 / 22 | –             |
+|           | padding       | 16 / 1   | `$spacing-05` |
+| Caret tip | height, width | 8 / .5   | –             |
+|           | margin-top    | 4 / .25  | `$spacing-02` |
+
+<div className="image--fixed">
+
+![Structure and spacing measurements for a popover container.](images/popover-style-3.png)
+
+</div>
+
+<Caption>
+  Structure and spacing measurements a popover container. | px / rem
+</Caption>
+
+<div className="image--fixed">
+
+![Structure and spacing measurements between a popover container and trigger button.](images/popover-style-4.png)
+
+</div>
+
+<Caption>
+  Structure and spacing measurements between a popover container and trigger
+  button. | px / rem
+</Caption>