Documentation

### Describe the feature

Vert.x JUnit 5 has been improved to facilitate and streamline asynchronous testing.

The documentation needs a serious rewrite and put emphasis on

1. the integration with Vert.x exception handler
2. JUnit 5 integration
3. Checkpoint as preferred method to complete a test, over `completeNow`

`VertxTestContext` should be deprecated and the documentation explain how to migrate to the new usage:

- `verify` is handled automatically when running on a vertx thread managed by Vert.x instance created by `@VertxTest`
- `completeNow` / `fail` is replaced by checkpoint being completable of object

Here is a contribution migrating Vert.x Web to this API demonstrating the simplified API: https://github.com/vert-x3/vertx-web/pull/2879/changes

New features should be documented too:

- `@VertxTest`
- `@ProvidedBy` / `VertxProvider`
- `Checkpoint#asLatch`