This commit updates the term 'stream' to 'channel' in the
section header 'Streams for Zulip users and administrators',
in /development-community'.
Also, references to this section has been updated.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
