Docs: Add 12-factor how-to sections for integrations

[erinecon]

Overview

Add three new sections on how to integrate a web app charm with database, ingress, or observability.

Note: These sections are limited in scope to integration only. Eventually we will add content about how to configure ingress and how to use observability (getting the IP address, visiting the Grafana dashboard, viewing logs, etc).

Summary of changes

1. Move the "In this example" content about integrating with a database to its own dedicated section.
2. Add a new section, "Integrate with ingress"
3. Add a new section, "Integrate with observability"