Updates the self-hosted and Zulip Cloud billing articles for features
that have been added to billing management such as paying by invoice,
adding billing contact information, viewing past invoices, etc.
Revises text to be cleaner and clearer in some places.
More consistently uses bold (when not linked) for plan names.
Fixes a few errors or omissions in existing instructions.
Co-authored-by: Alya Abbott <alya@zulip.com>
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.