Add docs for terminal chat

[Tyriar]

https://code.visualstudio.com/docs/copilot/copilot-chat

[ntrogh]

Copilot docs updated in https://github.com/microsoft/vscode-docs/pull/7258 Still needs cross-referencing from the terminal docs.

[Tyriar]

@ntrogh where were you thinking of doing cross referencing? I'm fine with just having it described in copilot docs.

[ntrogh]

@Tyriar I suggest to add a paragraph in the Terminal Basics article to inform people that there is such a thing as terminal inline chat. For more details about Copilot, we can point them to the Copilot Chat docs: https://code.visualstudio.com/docs/copilot/copilot-chat#_terminal-inline-chat