Updates to Alert and Alert integrations docs

What does this MR do?

1. Deleted sections on the Alerts page that talk about configuring alerts because this content exists on the Alert Integrations Page
2. Moved section on setting up Opsgenie integration from Alerts page to Alert Integrations page
3. Changed title of Alerts page
4. Added intro to Alerts page

Related issues

#234019 (closed)

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