[DOCS] Create task related to backing up your cluster

[lcawl]

Per elastic/stack-docs#226 (comment) the documentation about "Upgrading the Stack" (https://www.elastic.co/guide/en/elastic-stack/7.0/upgrading-elastic-stack.html) says:

Back up your data. You cannot roll back to an earlier version unless you have a snapshot of your data. For information about creating snapshots, see Snapshot and Restore.

That "Snapshot and Restore" is a reference page (https://www.elastic.co/guide/en/elasticsearch/reference/7.0/modules-snapshots.html) and it would be better to refer users to a task-oriented page like https://www.elastic.co/guide/en/elasticsearch/guide/2.x/backing-up-your-cluster.html which can walk users through a typical set of steps to back up their cluster.

See also elastic/stack-docs#226 (comment)

[elasticmachine]

Pinging @elastic/es-distributed

[debadair]

[docs issue triage]

[debadair]

Closing as fixed.