docs(Popover): add basic documentation (#8773)

* docs(Popover): add basic documentation

* docs(Popover): rename file

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: dakahn

packages/react/src/components/Popover/README.md

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+# Popover
+
+A popover is a piece of UI that _pops over_ or renders on top of other UI on the
+page. It's a component "primitive" meaning it can be used to help make more
+complex components that utilize this behavior.
+
+## Popover Behaviors
+
+- popovers are invoked/shown by interacting with some other UI element either
+  through click, hover, key press, or focus
+- popovers can be dismissed either through blur, <kbd>esc</kbd> keypress, or by
+  having a different popover be triggered and shown
+- popovers are displayed on top of other UI and out of the normal page flow
+  typically anchored to a triggering element
+
+## Further Reading
+
+- [Open UI Popup documentation](https://open-ui.org/components/popup.research)