docs: Fix README config section key mismatch and add full configuration example

[teesofttech]

Problem

The README shows "Tableau" as the configuration section key:

{
  "Tableau": {
    "ServerUrl": "https://your-tableau-server",
    ...
  }
}

But the code in IServiceCollectionExtensions.AddTableauSharp() binds to "TableauOptions" and "TableauAuthOptions" — two separate sections.

The README also omits the TableauOptions section entirely and uses a non-existent field name (ServerUrl instead of Server).

Fix

Update README to show the correct two-section appsettings.json structure matching what the code actually reads, including all fields with descriptions.

Affected File

README.md

[teesofttech]

Alignment note

Docs should reflect the final configuration shape chosen in #21. Update this after the auth/client foundation is settled so README examples do not preserve a temporary API.