New doc - .NET Agent ecosystem overview

[alexwolfmsft]

Describe the issue or suggestion

Create new content for the .NET AI docs that explain the current landscape for agent frameworks/options and when to use each. As we expand our agent coverage this will become increasingly important. Cover topics such as:

• Which agent frameworks are currently available, and a summary of each
• The relevant packages for each fraemwork
• When to use one framework vs another, and which scenarios they would be most useful.