Expand workspaces page

[bartlomieju]

As per denoland/deno#28894 (comment) there is a bit of missing information on the workspaces page, mainly:

• Deno supports workspace:*, workspace:~ and workspace:^ specifiers in package.json files
• Plain npm or pnpm workspaces work - for pnpm workspace it is necessary to migrate to deno.json instead of pnpm-workspace.yaml file