[zmq] add an introductive docstring on ZMQComm

[server config] on quick start, system source is still activated. Closes #2535714

[web] use `inline` `Content-Disposition` by default (closes #2535734) since known time we have been serving static file with a `filename` parameter on the `Content-Disposition` header. But since d74addac92bb we explicitly serve file as attachment if a filename is provided. However this is valid to have `inline` disposition and `filename` parameter. This changeset revert this part of d74addac92bb, going back to `inlined` by default. `IDownloadable` code explicitly request `attachment` content to preserve expected behavior.