Document how to do a "start up graph"

[hjoliver]

I get this question fairly often: how to make special stuff happen in the first cycle, before the next cycle starts.

If there's no inter-cycle dependence, some users find it surprising that upcoming cycles start running concurrently with the first cycle.

Example:

# First cycle only:
a => b => x

# Subsequent cycles:
x => y

# Caveat: there's no dependence between instances of `x`;  but none of them should
#   run before 1/b is finished, because it does some kind of prep needed by all `x`s.

If all xs do technically depend on b, as described, we should take that literally:

R1 = "a => b"
P1 = "b[^] => x => y"

(TODO: there used to be some downsides to perpetual dependence on the initial cycle point - for graph visualization, and cycle computation? - not sure if that still matters for Cylc 8).

Alternatively, use an artificial dependence between instances of x that makes them submit in order but still allows them to run concurrently:

R1 = "a => b => x"
P1 = "x[-P1] => x => y"

[hjoliver]

(We already document dependence on initial cycle point via foo[^] = bar, but I think it would be helpful to explicitly discuss the "start-up graph" concept).

[hjoliver]

See also: cylc/cylc-flow#4912

[hjoliver]

This should be done along with #511