Allow to define a keep together open block to avoid page break between related content

[ggrossetie]

The idea is to be able to create a "keep-together" block to avoid page break:

[.keep-together]
--
The following image shows the stellar evolution of a high-mass star:

image::star[]
--

In the above document, we want to avoid a page break between the introductory paragraph and the image.
Similarly, we could use this block with other types of content:

[.keep-together]
--
To install Antora, open a terminal and type:

 $ npm i -g @antora/cli@2.2 @antora/site-generator-default@2.2

IMPORTANT: The `@` at the beginning of the package name is important.
It tells `npm` that the `cli` package is located in the `antora` group.
If you omit this character, `npm` will assume the package name is the name of a git repository on GitHub.
The second `@` offsets the requested version number.
Only the major and minor segments are specified to ensure you receive the latest patch update.
--