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 <[email protected]>

docs/releasing.md

@@ -15,6 +15,13 @@ Every release asset, binary, installer product name and package uses the SolidCr
 copies, the deprecated old-name CLI alias); keep them until that document says they can go. Do not
 rewrite published assets, checksums, tags or historical release notes of earlier versions.
 
+The main installers (`.msi`, portable `.zip`, `.dmg`, `.AppImage`, `.deb`, `.rpm`, `.flatpak`) are
+also attached without the version in their names, for example `solidcraft-windows-x64.msi` and
+`solidcraft-linux-x86_64.AppImage`, and listed in `SHA256SUMS.txt`. The README's Download section
+links to `https://github.com/bherbruck/solidcraft/releases/latest/download/<name>`, which follows
+the release marked latest; nightlies never are. Keep these names stable: renaming one breaks the
+README links.
+
 Project Pages publishes the documentation, browser app and published-release Flatpak
 repository under `/solidcraft/`. Do not add a separate owner-site publisher or permanent
 old-name Pages/Flatpak aliases. GitHub repository redirects are separate from Pages hosting;