Add a more prominent link or admonition to conda-forge publishing of plugin

[willingc]

This is based on [this conversation on zulip}(https://napari.zulipchat.com/#narrow/channel/309872-plugins/topic/Can't.20find.20my.20plugin.20in.20napari.20bundled.20app.2E/near/479905131).

Doc page where I missed the admonition box about Conda when I was scanning the page for conda-forge: https://napari.org/dev/plugins/testing_and_publishing/deploy.html#plugin-deploy

For my eyes, the Attention admonition https://napari.org/napari-sphinx-theme/test1.html#admonition-colors would be most readable when scanning.

cc/ @melissawm