docs: added docs for examples

examples/comprehensive/tiny/README.md

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+# Tiny Example
+
+This exmaple shows the smallest possible and meaningful Perseus app, a simple 'Hello World!' app. This is considered a comprehensive example because it doesn't show a particular feature of Perseus, nor does it demonstrate how to do a common task. The structure of this example isn't likely to be used anywhere in the real world (your templates and error pages would nearly alwasy be in separate files), but it serves as a great way of showing the absolute basics of Perseus and getting new users to hit the ground running.

examples/core/basic/README.md

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+# Basic Example
+
+This is a basic example of Perseus that shows the fundamentals of a slightly more advanced Perseus app. This is considered a core example because it not only contains end-to-end tests for Perseus itself, it's also the site of the development of the default Perseus engine. For that reason, the `.perseus/` directory is checked into Git here, and this is used as the single source of truth for the default engine. The rason for developing it here is to provide the context of an actual usage of Perseus, which makes a number of things easier. Then, some scripts bridge the gap to make the engine integrate into the CLI (you shouldn't have to worry about this when working in the Perseus repo as long as you're using the `bonnie dev example ...` script).

examples/core/i18n/README.md

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+# Internationalization Example
+
+This example shows a very basic Perseus app using internationalization (abbreviated *i18n*) in three languages: English, French, and Spanish. This shows how to use translations, access them, and how to insert variables into them.
+
+Note that this i18n in this example uses the [Fluent](https://projectfluent.org) integration.

examples/core/plugins/README.md

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+# Plugins Example
+
+This example shows how Perseus' plugins system works by creating a very simple plugin and applying it to a simple app.