Fix broken annotation label

[Incogdino]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Closes #2563

Overview of changes:
After investigating #2563, the image annotation started breaking after converting to use the slots API.

According to Vue's documentation, content passed by the slots API are considered to be owned by the parent component that passes them in and so styles do not apply to them.

A separate CSS rule to target all elements within the div that holds the slot is added to override the parent's styles. This fixed the issue and the problem no longer persists.

Some screenshots:

Anything you'd like to highlight/discuss:

Testing instructions:
Testing can be done locally by navigating to the docs folder and running npm markbind serve -d -o userGuide/components/imagesAndDiagrams.md

Proposed commit message: (wrap lines at 72 characters)
Fix broken annotation label

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[Tim-Siu]

Thanks @Incogdino for investigating this issue. The investigation is sound. It is also mentioned in [the documentation](https://vuejs.org/guide/components/slots) that "The slot mechanism is declarative: the component cannot directly read or transform the slot content". The fix works for me with both safari and chrome on macOS.

[Tim-Siu]

LGTM. This modification targets only the hover-label and should be a safe fix.