docs: clarify charm icon handling across plugins

[gcomneno]

Summary

Add a note to the charm management docs clarifying that icon.svg handling depends on the Charmcraft plugin or profile in use.

The note calls out that modern charms in this repository typically use the uv plugin, while older charms may use dump, so the behavior is not identical in every case.

Testing

• make -C docs html
• make -C docs vale TARGET='docs/howto/manage-charms.md'
• make -C docs spelling TARGET='docs/howto/manage-charms.md'

[dwilding]

Thanks for the addition @gcomneno! Just a heads up - I won't review or merge this until I've finished #2420, which affects where we might put advice about icons.

[gcomneno]

Thanks for the heads up! That makes sense. I’ll leave this as-is for now and wait for #2420 to settle before making any further changes.

[tonyandrewmeyer]

This is also not quite correct. If you're using uv for the charm plugin, then you need to use the dump plugin to include the icon file. I think that is the case for poetry and python too, but I'm not 100% sure on that. I think the old charm plugin is the only one that pulls in the icon by default.

I think we would also want to have an example of what the YAML would look like to add the icon.

[gcomneno]

Thanks Tony, that's helpful. I agree the current wording is too broad.

I'll hold off on revising this until #2420 settles where this advice should live, but the next version should distinguish the old charm plugin from uv/poetry/python-based projects and include a concrete YAML example using the dump plugin to include the icon file.