docs: add clarification on theme ejection

[peterhartree]

Figuring this out was the first pain point for me in getting started with VuePress. So I think maybe above bar to add a line like this.

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
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If changing the UI of default theme, please provide the before/after screenshot:

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Other information:

[bencodezen]

Thanks for the PR @peterhartree! Great addition!