[REQUEST][ResponseOps][Alerting] Alert Deletion Docs

[ymao1]

Scope
We are adding a new alerts history clean-up as part of the stack management rules settings flyout to trigger a one time manual bulk alerts deletion

User flows
Users can trigger a one time manual bulk alerts deletion by navigating to Stack management -> Rules -> Settings. There are two conditions:

• Users can set up a one time alert deletion run based on whether if the alerts are active/inactive
• Users can set up a one time alert deletion run based on the age of the alert

References
GH issues:

• [Response Ops][Alerting] Implement alert deletion task kibana#208615
• [Response Ops][Alerting] Implement alert deletion UI kibana#208621

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[ymao1]

cc @lcawl

[tiamliu]

cc @nastasha-solomon