Describe the VertxInstance service in the documentation and on the FAQ of the website

[DavideD]

The VertxInstace class is quite critical when using Hibernate Reactive.

We don't mention it anywhere in the documentation.
We should explain what it does and how to use it.

[DavideD]

Here's some of the information that we need to include: #750
We should also add them in a FAQ section on the website.

[gavinking]

Might be worth mentioning the new Context interface too, which has some sort-of useful operations, like execute().