The "Before it all starts" section wasn't very clear in its purpose
and scope, merge it with the "File layout" section.

It seems like the `htools:iextags:FOO` parameter is a PREFIX. So the
previous system didn't actually work, because (e.g.) no web-fsn:* tag
existed.

I have fixed the actual cluster to use service:* tags.

See team#29816

The rationale here is that Ganeti has its own inventory and capacity
evaluation system, so we do not *need* the Spreadsheet for that
purpose. We still do need the spreadsheet for cost tracking however,
so keep the documentation procedure for machines that "are not in
Ganeti", basically.