suggestion: document workspaces functionality

This is important as the Deno Standard Library will become a workspaces-enabled codebase once it is fully ported to JSR. Documenting how workspaces work will be important to those maintaining and contributing to it.

Some documentation on workspaces functionality can be seen in #245.

This documentation should contain information on both workspaces setting in deno.json as well as using "workspaces" in VSCode

JSON schema is not suggesting workspaces setting in the editor. @nathanwhit is working on that

Closed by https://github.com/denoland/deno-docs/pull/567