Publish release candidates to a testing apt distribution

Pre-releases (versions with a hyphen) now upload their .debs to the
"testing" distribution instead of "stable", so servers following stable
never get an RC from apt upgrade. An RC can take its release notes from
its final version's section or Unreleased; the workflow checks the notes
with 'gitea-release.sh notes'. Also fix publishing RCs, whose .deb names
carry the Debian version (0.2.0~rc.1) and were rejected.

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

docs/install.md

@@ -60,6 +60,17 @@ sudo apt update
 sudo apt install jarl
 ```
 
+Release candidates (`0.2.0-rc.1`) are published to a separate `testing`
+distribution, so the `stable` line above only ever installs releases. To
+try release candidates on a test machine, add `testing` as well. apt then
+installs the newest of either, which is the RC until the final release
+replaces it:
+
+```sh
+echo "deb [signed-by=/etc/apt/keyrings/jarl.asc] https://gitea.adbraun.lol/api/packages/adam/debian testing main" \
+  | sudo tee -a /etc/apt/sources.list.d/jarl.list
+```
+
 The packages are as visible as the Gitea account that owns them, not the
 repository: with a public account, apt needs no login even if the
 repository is private. For a private or limited account, give apt a token