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.

We add the Next label because it's already being used in a subproject
of tpa (gitlab) and because it's used globally. It also helps us
separate the "endless pile of ticket" in Open from the Backlog, as
now we can throw more things in the backlog without worrying about
overloading our monthly capacity.