docs: remove tutorial chapters that are covered by the how-to guide

[tonyandrewmeyer]

The (k8s) tutorial is currently quite long, and we'd like it to be shorter so that it's easier for charmers to get going.

The plan is that the features described in the tutorial should be the chapters. Namely:

• A simple web server with endpoints that can be used, including metrics
• Integration with postgresql
• An action to be able to manually interact with postgresql
• COS integration, including logs and metrics

This PR removes the chapters:

• Exposing the workload version (the existing how-to guide already covers this, and it's not essential for a charm)
• Storing state (we would generally prefer that charms don't do this, so shouldn't encourage it, and it'll be covered by a how-to guide before this PR is ready for review)
• Exposing a port via K8s (too small for a tutorial chapter, and the existing how-to guide already covers this - and was based mostly on this chapter, in the knowledge that it'd be replacing it)
• Write Harness tests - the new unit testing (Scenario) chapter is merged into this. The testing chapters need to be migrated into sections at the end of each chapter, but that's work for another day and another person. This just stops teaching new charmers to write Harness tests

There are a few other small tidy-ups, particularly around whitespace and removing old commented text.

[tonyandrewmeyer]

The k8s-tutorial-sdk repository will also need to be updated, including renaming a bunch of chapters, in sync with this getting merged.

I'd suggest that we don't merge this until the Discourse versions go away, if possible, so that the example repo can always match the live version.

[benhoyt]

Should we get rid of the "nn_" prefixes on the branch names to avoid having to rename things (more than the once)?

[tonyandrewmeyer]

I wondered about that too. Adding (especially inserting) or removing a chapter is annoying, because there's this chunk at the start, plus a similar one at the end, as well as previous/next links - and this repo and the tutorial one need to be kept in sync. I think we can probably automate a bunch of that now that we don't have to work with Discourse.

On the other hand, it is nice to have a clear order, and that's missing from the docs now that they don't have leading numbers. We probably aren't adding/removing pages all that often.