docs: remove comments from K8s tutorial and clarify about persisting data

[dwilding]

The K8s charm tutorial contains comments with original/draft text of the tutorial. For the most part, the comments are captured in the text of the tutorial. I've reviewed all the comments and don't think we need to keep any of them.

While doing this, I felt it was worth expanding one tip in Create a minimal Kubernetes charm. I've used similar wording to the machine charm tutorial.

Resolves #2070.

[Copilot]

Pull request overview

This PR removes outdated HTML comments from the Kubernetes charm tutorial documentation and enhances guidance about data persistence in charms. The comments contained draft text that has been incorporated into the published tutorial and is no longer needed.

Key changes:

• Removed all HTML comment blocks containing draft/original tutorial text across multiple documentation files
• Expanded the tip about charm initialization in the "Create a minimal Kubernetes charm" section to clarify that data cannot be persisted between Juju events by storing it in memory

Reviewed changes

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

File	Description
study-your-application.md	Removed HTML comments containing draft application overview text and Docker build instructions
observe-your-charm-with-cos-lite.md	Removed HTML comments with draft text about Grafana dashboard links and API endpoint testing
integrate-your-charm-with-postgresql.md	Removed HTML comment about updating documentation links
create-a-minimal-kubernetes-charm.md	Removed HTML comments with draft explanations and expanded the tip about charm initialization to clarify data persistence limitations

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[dwilding]

These details are captured in the README of the demo server repo: https://github.com/canonical/api_demo_server