docs: clarify handling of postgres relation data in K8s tutorial

[dwilding]

In Integrate your charm with PostgreSQL, the part from "Add relation event observers" to the end of "Share the authentication information with your application" needs improving:

• I find the sequence of file edits confusing. I don't know what the goal is until I get to the end and work backwards.
• A couple of the method explanations and docstrings are much wordier than they need to be.
• A couple of places still talk about setting status without collect-status: The diagram at the end and the part mentioned in #2095.

I've reorganised the flow and improved the explanations of what's happening. That includes replacing the diagram by more focused text.

Previews of updated sections:

• Add relation event observers
• Fetch the database authentication data
• Share the authentication data with your application

[Copilot]

Pull request overview

This PR improves the PostgreSQL integration tutorial by reorganizing the documentation flow and simplifying docstrings. The changes make it easier to understand how to handle database relation data by presenting steps in a more logical sequence and reducing verbose explanations.

Key changes:

• Reorganized tutorial sections to present database authentication handling before Pebble layer updates
• Simplified overly verbose docstrings in get_app_environment() and fetch_postgres_relation_data() methods
• Replaced a diagram with clearer textual explanation of service behavior with/without database relations

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
docs/tutorial/from-zero-to-hero-write-your-first-kubernetes-charm/integrate-your-charm-with-postgresql.md	Reorganized tutorial flow to introduce database authentication methods before Pebble layer updates; replaced diagram with text explanation
examples/k8s-3-postgresql/src/charm.py	Simplified docstrings for get_app_environment() and fetch_postgres_relation_data() methods
examples/k8s-4-action/src/charm.py	Simplified docstrings for get_app_environment() and fetch_postgres_relation_data() methods
examples/k8s-5-observe/src/charm.py	Simplified docstrings for get_app_environment() and fetch_postgres_relation_data() methods

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