Microsoft has been renaming AzureAD to Entra ID. Though both names still
seem to function, even if unofficially, so this mostly renames to `Entra
ID (AzureAD)` to reference both for clarity.
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.
