Add web URL for context.

## Problem

I want to be able to just pass a reference URL such as a particular docs website as context for Amazon Q in vscode.

## Expected behavior

When using @ or pin context, one option should be URL or web and then you pass the link and it works like other files do.
Should at least work openly available websites - not necessarily internal stuff

Start by tracing the existing @ and pin context flow in the VS Code extension and how file context is passed to Amazon Q. Define the public-website boundary and confirm that done means a URL or web option accepts an openly available docs link and makes it work like file context.

Written by the indexing model from the issue text.