[Feature Anywhere] [Meta] Add documentation for Feature Anywhere

[ohltyler]

Coming from #2880

We will want to have clear documentation in READMEs to show the following:

• purpose of the vis_augmenter plugin and how it works
• examples of each added data model (a VisLayer, a PluginResource, an augment-vis saved object, a vis_layer expressions function)
• detailed steps on how to onboard a plugin. This should live in the vis_augmenter README, as well as a standalone blog. It may reference plugin documentation with examples
• examples, usage, & purpose of the new vis_augmenter/stats API
• limitations / gotchas of the feature (e.g., default limit of 10 resources per vis)
• the loose coupling, such as freedom to make visualizations ineligible, add plugin resources that are unrelated to the vis, etc.