Add permalink spacing handling to subnav when its sticky

[jescalan]

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Description

When the subnav is in sticky mode, and you have a permalink on the page, the headline ends up scrolling underneath the menu. This little css thingy prevents that from happening. It used to be implemented on each of our documentation sites hanging off the global .g-subnav but that no longer works because of css modules, and honestly it should have been part of this component anyway since I can't really imagine a scenario where you would want your permalinks to be hidden by this component, so I think this should do the trick. Tested on vault docs website and it worked wonderfully.

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🦋 Changeset detected

Latest commit: a081f03

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@hashicorp/react-subnav	Patch

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[BRKalow]

Nice, thanks. I had seen this during the upgrade and was wondering what exactly it was for.