docs: grab workload version in k8s tutorial

[tromai]

Closes #2282.

I open this as a draft for an initial review of the structure and direction of this update. My changes now are for the first tutorial chapter: creating a minimal k8s charm. After the content is finalized, I will port them to the remaining chapters.

[benhoyt]

Let's avoid autouse fixtures unless absolutely necessary, as they can be quite confusing action-at-a-distance. Something like:

@pytest.fixture
def fake_version(monkeypath):
    monkeypath.setattr(...)

def test_pebble_layer(fake_version):
    ...

[benhoyt]

It's hard to tell what's being added here. Could we consider something like this?

Suggested change

	def _on_demo_server_pebble_ready(self, event: ops.PebbleReadyEvent) -> None:
	"""Define and start a workload using the Pebble API."""
	# Get a reference the container attribute on the PebbleReadyEvent
	container = event.workload
	# Add initial Pebble config layer using the Pebble API
	container.add_layer("fastapi_demo", self._get_pebble_layer(), combine=True)
	# Make Pebble reevaluate its plan, ensuring any services are started if enabled.
	container.replan()
	# Set the workload version of this charm.
	version = fastapi_demo.get_version(port=8000)
	self.unit.set_workload_version(version)
	# Learn more about statuses at
	# https://documentation.ubuntu.com/juju/3.6/reference/status/
	self.unit.status = ops.ActiveStatus()
	def _on_demo_server_pebble_ready(self, event: ops.PebbleReadyEvent) -> None:
	# ... previous lines ...
	# Set the workload version of this charm.
	version = fastapi_demo.get_version(port=8000)
	self.unit.set_workload_version(version)

[benhoyt]

Consider using something a bit more general, to avoid updates if the demo-server is updated with a minor or patch release:

Suggested change

	assert version == "1.0.4"
	assert version.startswith("1.")