make another section for other alternatives

the rationale here is those were not explicitely rejected, but more
that they are out of scope or were found later.

howto/conference.md

@@ -308,7 +308,10 @@ Media](https://tech.firstlook.media/how-to-pick-a-video-conferencing-platform) p
  * [Zoom](https://zoom.us/) - paid service, serious server and client-side security
    issues, not E2EE, but very popular and fairly reliable
 
-Conference-hosting and larger-scale e-learning software:
+### Other alternatives
+
+Those alternatives have not been explicitely rejected but are somewhat
+out of scope, or have come up after the evaluation was performed:
 
  * [bbb-scale](https://gitlab.com/bbb-scale/bbb-scale) - scale Big
    Blue Button to thousands of users