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/v0.6.0.md

@@ -0,0 +1,19 @@
+# v0.6.0 — Export to STL
+
+## ✨ New features
+
+**STL export with a settings dialog**
+* Save the parts on screen — including any alignment or rotation you applied — as an STL mesh for 3D printing or other software.
+* Before exporting you choose: one merged file or one file per input, Binary or ASCII, mm / inch / custom scale, and mesh quality (current or fine).
+* The dialog shows the triangle count and estimated file size live, so you know what you'll get before you hit export.
+
+## 📦 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:** [v0.5.0...v0.6.0](https://github.com/ETWen/STPViewer/compare/v0.5.0...v0.6.0)