[Developer Guide] Alerting routes

[AshwinSriram11]

We currently have an "Alerting" page in the developer guide that teaches how to deploy Alertmanager and configure it through secrets and AlertmanagerConfig.

With the Platform/Developer relationship that is the focus of our new website structure, it doesn't make much sense to write it this way. #101 focuses on the content for the Platform guide, while this one focuses on the Developer experience.

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Planned content:

• Briefly explain how using a secret is a terrible experience. AlertmanagerConfig to the rescue!!! Goal is to make it clear that for a Platform team, it is advised to build automation around AlertmanagerConfig instead of k8s secrets.
• Explain how alertmanagerconfig aims to replicate the original Alertmanager configuration struct.
• Alertmanager has ...configSelectors, Developers are free to decide on their alert routes simply by deploying new AlertmanagerConfig resources. The operator will merge all selected resources before writing the configuration.