docs: ✏️ Add documentation for pnpm overrides (#3263)

ui/desktop/electron-app/pnpm-overrides.md

@@ -0,0 +1,9 @@
+# package.json Overrides
+
+This file documents the rationale for each dependency override defined in `package.json` under `pnpm.overrides` for the `electron-app` sub-package.
+These overrides force specific versions of transitive dependencies, primarily for security and compatibility reasons.
+
+| Package | Version | Pulled in by | What to check before removing |
+|---------|---------|--------------|-------------------------------|
+| `tar` | `^7.5.10` | Both `cacache` (`^6.1.11`) and `@electron/rebuild` (`^6.0.5`) pin old ranges. | Check whether newer releases of `@electron/rebuild` and `cacache` (brought in via `@electron-forge` and `electron`) allow `tar ^7.x`. |
+