uui-popover-container: set popover attribute automatically

[JesmoDev]

This PR makes sure that the popover container always has the popover attribute as it is needed by the native Popover API.
It checks if a popover value is already present (example popover='manual') to prevent unwanted overides.
Also updates the docs to reflect the changes

Now you can use the popover as such:

<uui-popover-container id="my-popover">
...content
</uui-popover-container>

instead of

<uui-popover-container id="my-popover" popover>
...content
</uui-popover-container>

Description

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

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