docs: add release notes for v0.1.0–v0.4.0, v0.6.0 and v0.7.0

Backfill notes for versions that had a tag but no release page, so
every version up to v0.7.1 has a GitHub/Gitea release. Tag v0.7.0 at
45b27d5 (was missing).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/release-notes/v0.3.0.md

@@ -0,0 +1,22 @@
+# v0.3.0 — 3-axis gizmo for precise move & rotate
+
+## ✨ New features
+
+**Gizmo manipulator**
+* Select a file in the tree and a gizmo appears at its center: drag the X/Y/Z arrows to move exactly along one axis, or the rotation rings to spin it by any angle — from any viewing angle.
+* Selecting another file moves the gizmo to it.
+
+## ⚠️ Known limitation
+
+* The gizmo can be hidden inside solid parts. (Fixed in v0.3.1 — the gizmo now always stays on top.)
+
+## 📦 Downloads
+
+Source only — no prebuilt binaries were published for this version. For a ready-to-run build, grab the [latest release](https://github.com/ETWen/STPViewer/releases/latest).
+
+## 🔗 Links
+
+* [Architecture](https://github.com/ETWen/STPViewer/blob/main/ARCHITECTURE.md)
+* [README](https://github.com/ETWen/STPViewer/blob/main/README.md)
+
+**Full changelog:** [v0.2.1...v0.3.0](https://github.com/ETWen/STPViewer/compare/v0.2.1...v0.3.0)