Update Supported Provider Doc

[npolshakova]

The website page for configuring providers (https://kagent.dev/docs/getting-started/configuring-providers) is a little out of date:

• Update the list of providers are currently supported
• Remove the link to the closed issue ((note that there's an active issue with tool calling
  ))

[artberger]

I think most had been added since this issue was opened except Gemini, in this PR: #176