[Popover] Don't continuously observeRect on Popovers that are hidden

[cpmsmith]

This pull request:

• Fixes a bug in an existing package
• Updates documentation or example code

Fixes #636.

Since, in order to have an aria-controls attribute, MenuButton merely hides its Popover rather than not rendering it, the Popover is constantly running getBoundingClientRect by way of useRect whenever there's a MenuButton on the page. This change makes Popover pass the observe parameter to useRect according to its hidden prop, and adds a storybook example for demoing it.

I added the storybook in order to try and break it, i.e. by changing the location of the target while observe was false, and couldn't manage to do so.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 85c1e85:

Sandbox	Source
reach/reach-ui: reach-ui	Configuration

[cpmsmith]

Here's a trace of the basic MenuButton storybook example before the change:

And after, with the menu open from ~2.7–5.0s:

[sciyoshi]

@chancestrickland @ryanflorence anything left before this PR can be merged? We've been running into this and it causes some performance issues - having a way to hide the popovers when not rendered would be great.