Add cheat sheet entry on rose date -> isodatetime

[MetRonnie]

Paired with cylc/cylc-flow#5838

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.

[MetRonnie]

Tests failing in same way on master - fixed by cylc/cylc-flow#5718

[ukmo-ccbunney]

Just a comment : I like the syntax synopsis as you have defined it in the linter with the square brackets as this makes it clearer that the ref is replacing the -c, i.e.

rose date [-c] => isodatetime [ref]

[MetRonnie]

Slightly changed for clarity:

@ukmo-ccbunney: Just a comment : I like the syntax synopsis as you have defined it in the linter with the square brackets as this makes it clearer that the ref is replacing the -c, i.e.

rose date [-c] => isodatetime [ref]

This was done for brevity as the -c/ref is optional. Here, I think it's fairly clear with how the rows line up?

[ukmo-ccbunney]

Looks perfect @MetRonnie .