chore: Prepare https://arrow.apache.org/dotnet/

[kou]

What's Changed

Add support for publishing to https://arrow.apache.org/dotnet/ :

• The main branch -> https://arrow.apache.org/dotnet/main/
• The vX.Y.Z tag -> https://arrow.apache.org/dotnet/X.Y.Z/
• The latest vX.Y.Z tag -> https://arrow.apache.org/dotnet/current/
• https://arrow.apache.org/dotnet/ -> https://arrow.apache.org/dotnet/current/

Add support for previewing on fork: https://${GITHUB_ID}.github.io/arrow-dotnet/
Example: https://kou.github.io/arrow-dotnet/

See docs/README.md how to prepare fork to preview.

Add support for commenting preview URL on PR.

Closes #26.

[adamreeve]

Looks good thanks @kou, just some minor suggestions

[kou]

Thanks! I've applied all your suggestions!