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
rose bush jobs: fix status for retried jobs

A failed job to be retried should be marked failed.
Merge pull request #1378 from matthewrmshin/fix-rose-bush-jobs-retry

rose bush jobs: fix status for retried jobs
rose host-select: improve performance

Don't SSH all hosts if ranking method is random and there is no
thresholds to consider. Shuffle host list, and return 1st available
host.