docs: add Downloads page + telnet re-migration workaround for #493

Address the two docs follow-ups from #493 (radio sources not mounting
after an in-place migration).

Troubleshooting: the "Radio sources never activate after an in-place
migration" entry now leads with the confirmed non-destructive fix,
re-running migration via the telnet method so all four service URLs
(incl. bmxRegistryUrl / statsServerUrl) land on the speaker's runtime:

    soundtouch-cli --host <ip> setup migrate --method telnet --service-url http://<host>:8000

Factory reset is kept as the fallback for models without a reachable
telnet port. The cause text is updated to the diagnosed BMX-registry
explanation, and notes why pointing the service at http://bose:8000 with
a server-side /etc/hosts entry does not help.

Downloads: new top-level docs section (docs/content/docs/downloads/)
structured by tool (service / player / cli / backup) x OS/arch, using
the real release asset naming (soundtouch-<tool>-v<ver>-<os>-<arch>),
plus install-script, Docker, and go-install routes. Sibling section
weights bumped so Downloads leads the sidebar. README, the release-notes
template, and the key install guides now point here. Also fixes the
stale, never-produced .tar.gz/.zip filenames in SELF-HOSTING.md.

refs #493

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -362,6 +362,8 @@ jobs:
           - **soundtouch-cli**: command-line control of any device: playback, presets, sources, multiroom zones, discovery, and migration. Good for scripting and home automation.
           - **soundtouch-backup**: back up your Bose cloud account and each speaker's local state. \`soundtouch-backup all\` captures everything in one step.
 
+          Not sure which file to grab? The [Downloads page](https://gesellix.github.io/Bose-SoundTouch/docs/downloads/) explains which tool you need and which \`<os>-<arch>\` build matches your computer.
+
           ## Documentation
 
           Full guides, setup walkthroughs, and troubleshooting: https://gesellix.github.io/Bose-SoundTouch/