Document non-simultaneous use across multiple vscode installations

[jlrobins]

Document in extension readme shown on marketplace that the extension has been tested and installable in mainline vscode, vscode preview edition, and perhaps also Cursor (when that works), but clearly indicate that users can only run one at a time due to the sidecar process at a fixed port number.

Depends on:

• Make VSCode Extension URI a property of the Connection spec's CCloud config ide-sidecar#288
• Provide env.uriScheme to CCloud connections' ccloud_config #886