stasis_broadcast: Add StasisBroadcast documentation

Add a guide covering the new StasisBroadcast module:

- Introduction-to-ARI-and-Broadcast/index.md — overview with Mermaid
  sequence diagram, required modules table, step-by-step description,
  comparison table vs Stasis(), and Next Steps links.
- Introduction-to-ARI-and-Broadcast/Claiming-Broadcast-Channels.md —
  practical setup guide covering http.conf/ari.conf/modules.conf
  configuration, application parameters, STASISSTATUS values, CallBroadcast
  and CallClaimed event schemas, POST /ari/events/claim response codes,
  complete Python and Node.js examples, and advanced topics (filtering,
  channel variables, timeout/fallback, args).
- Introduction-to-ARI-and-Broadcast/extensions.conf — annotated sample
  dialplan covering all common StasisBroadcast() patterns.

Documentation Generated by Claude Code and reviewed by Author

Author: danieldonoghue

.gitignore

@@ -3,3 +3,4 @@ site
 Makefile.inc
 Makefile.*.inc
 temp
+.devcontainer/