Add documentation for hard_reset_node API

As part of https://issues.couchbase.com/browse/MB-55697,

We are adding a hard reset API call that will allow a user to reset a node to a pristine state.

This needs to be documented.

The hard reset API:
curl -X POST -u Administrator:asdasd 127.0.0.1:9001/controller/hardResetNode
resets the node at 127.0.0.1:9001 such that all state - both config and data are lost.

As such, the user should exercise caution before running a hard reset. They are strongly advised to take a backup because the node will be reset to a pristine state - all data on the node will be lost.

This API is intended primarily to be able to add a node back to a cluster after it has gone through unsafe failover. After a node had gone through unsafe failover, there was no convenient way to add it back to the cluster it was originally booted from. With the hard reset API, after an unsafe failover, a hard reset of the node will allow it to be added back to the cluster.

It requires full admin permission.