SingleSourceOfTruthViolation in realtime database change broadcast/subscription guides

# Improve documentation ## Link - https://supabase.com/docs/guides/realtime/broadcast#broadcast-from-the-database - https://supabase.com/docs/guides/realtime/postgres-changes - https://supabase.com/docs/guides/realtime/subscribing-to-database-changes ## Describe the problem The last guide contains code snippets from both the first and second guide while ignoring many information (such as that Broadcasts from the database are in public alpha). If anything changes e.g. in the code of the two first guides, it's likely that the third is forgotten (or vice versa). I now need to read both to understand realtime because I can't be sure that the third doesn't contain information which isn't present in the first/second. ## Describe the improvement I've two possible solutions in mind: - The last guide should be reviewed for information which aren't in the first two guides, these information should be added to the other two, then the guide should be removed OR - At least the code snippets should be extracted so that they are reused between all the guides.