README: put a Download section with per-platform installers at the top (#411)

Badges and one-line instructions for Windows, macOS, Linux and the web app,
right after the tagline and demo. The links use releases/latest/download with
version-less file names, which release.yml now attaches (and checksums)
alongside the versioned installers. The Flatpak note and the name/compatibility
section move into the book's Getting started page.


Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/release-notes.md

@@ -36,6 +36,8 @@ old-name Pages addresses are not permanent aliases.
 - **FreeBSD:** `tar -xzf solidcraft-{version}-freebsd-x86_64.tar.gz --strip-components 1 -C /usr/local`.
 - **Web:** `solidcraft-web-{version}.zip` is a static site to host yourself.
 - `SHA256SUMS.txt`: `sha256sum -c SHA256SUMS.txt --ignore-missing`.
+- Files without a version in the name (`solidcraft-windows-x64.msi`, `solidcraft-linux-x86_64.AppImage`, …)
+  are the same packages under fixed names, for download links that always fetch the latest release.
 
 For Flatpak feed installation and updates, add Flathub for the required runtime before
 installing SolidCraft; keep both remotes in the same user installation: