chore(theme): add AlgoliaSearchBox placeholder on loading

[benjamincanac]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Resolves #77

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/nuxtlabs/docus/A911y9ffvD9U5H11G45s4RUbM9cU
✅ Preview: https://docus-git-task-77-nuxtlabs.vercel.app