fix(github-edit-link): use source and contentDir to resolve edit link

[farnabaz]

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 #363

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/AeYCFDUfcNvEXpPfTMRn2R9bPBUV
✅ Preview: https://docus-git-fix-github-edit-nuxtlabs.vercel.app

[Tahul]

LGTM 😄