Write a new "Disaster Recovery" section in user-guide/running-workflows

[wxtim]

Check and re-write running-workflows/disaster recovery for Cylc8

[MetRonnie]

Old disaster recovery guide for reference: https://cylc.github.io/cylc-doc/8.0b1/html/user-guide/running-suites.html?highlight=disaster%20recovery#disaster-recovery

[oliver-sanders]

The 8.0b1 docs (linked to above) are now gone, here's the section as it was at 7.9.3

https://cylc.github.io/cylc-doc/7.9.3/html/running-suites.html?highlight=disaster%20recovery#disaster-recovery

[MetRonnie]

Or here for the 8.0b1 version: https://github.com/cylc/cylc-doc/blob/8.0b1/src/user-guide/running-suites.rst#disaster-recovery (which I've just checked looks pretty identical)