Promote new features when they are added to the plugin

[westonruter]

Feature Description

When first activating the plugin, there is an admin pointer encouraging users to go to the Performance screen. However, after they go there the first time and activate features (facilitated by #1032), there is nothing that currently draws users' attention back to the Performance screen, in particular when a new feature is added to the plugin. I see two ways this could be implemented:

1. Add a new admin pointer for the first release in which a feature appears. This admin pointer would only be shown if they haven't already dismissed the general admin pointer for onboarding.
2. Add a more subtle 🔴 dot next to the "Performance" menu item as a less obtrusive admin pointer.

The challenge with the second option is that users may not hover over the Settings menu or navigate to one of the settings pages to then see that red dot:

It would be most natural for the notice to be shown immediately after upgrading the plugin. However, with shiny updates this isn't really feasible.

This is relevant to the next release as I hope to include Image Prioritizer (#1088) as an experimental feature.

[westonruter]

Another option is the Upgrade Notice as an even less obtrusive way to promote new features.

The Web Stories plugin has an upgrade notice for the current version.

It gets displayed with the plugin on the Upgrade screen:

The concern here is that this Upgrade Notice is only shown on the Upgrade screen. It isn't shown on the plugin list table when doing shiny updates.