Docs: convert to ISO 8601

A first attempt.
Docs: remove cold start references

And other improvements.
Fix tutorial suite syntax

Was missing `graph=`.
tutorial doc: update `rose date` offset syntax
Tutorial docs: fix duration syntax
Fix example suite syntax spacing
Tutorial doc: ISO 8601 clock-triggered syntax
Doc: update for cylc 6

Fix directory path for `work/`.
Remove unnecessary comments.
Merge pull request #1374 from matthewrmshin/doc-iso8601

 Docs: convert to ISO 8601