tools: add update_gitea_releases.py and release-notes snippets

One-shot script to backfill Gitea release bodies for existing releases
using the Gitea API. Reads per-version Markdown files from
tools/release-notes/ and PATCHes each release body.
Includes README.md in tools/release-notes/ explaining usage.

tools/release-notes/v2.1.0.md

@@ -0,0 +1,13 @@
+### Added
+- NSIS plugin API (`pluginapi.cpp`/`pluginapi.h`) for 7-Zip zstd build
+- `ExtractCallbackConsole` class for extraction progress and user input
+- Main extraction logic (`Main.cpp`, `MainAr.cpp`) for the zstd bundle
+- User input utilities (`UserInputUtils2`) for password handling and prompts
+- Break signal handling (`NSISBreak`) for graceful interruption
+- `Nsis7z_vs2026.vcxproj` (v145 toolset) for Visual Studio 2026 builds
+- `tools/fix_vcxproj.py` helper for project file patching
+- `versions/7-zip-zstd` submodule added
+- `build_zstd.cmd` top-level build script for the zstd variant
+
+---
+_Repository primario: vedi link Gitea nel README. Il mirror GitHub è generato automaticamente._