Update samples and docs to add Host.CreateDefaultBuilder()

With the recent addition of app insights logging integration on dotnet worker, we should consider exploring the idea of adding an `appsettings.json` file as a default configuration source to isolated function apps. For customers who are coming from aspnet core webapi background, this experience makes it easier for them.

- Update new project template to include the file.
- Should we have environment specific files (`appsettings.{Environment}.json` ) ?

Locate the new project template, affected samples, and documentation referenced by the issue, then review how isolated function apps currently obtain configuration and how Host.CreateDefaultBuilder() is used. Resolve whether appsettings.json and appsettings.{Environment}.json should be included, update the template and docs accordingly, and verify the generated project and samples reflect the decision.

Written by the indexing model from the issue text.