Clearly document usage of internal files

[abelsromero]

Thank you for taking your time to talk with us!

What is this issue about?

• Bug report
• Feature request
• Question
• Documentation

Description
Usage of internal files (prefixed with _) can be better exposed in the docs.
Current explanation in a simple admonition that can pass unnoticed. As suggested in #545 (comment) it's much better if have an specific section.

That means either putting this information in the preamble, or a section above "Setup" 
named "How files are processed". There, it can say that this plugin looks for all AsciiDoc 
files in the specified directory, but ignores files that are considered non-public, such
 as internal include files and hidden files.

This section can be set in a partial file to be reused for all goals.

For clarification: current behaviour was inherited from AsciidoctorJ https://github.com/asciidoctor/asciidoctorj/blob/main/asciidoctorj-core/src/main/java/org/asciidoctor/jruby/AsciiDocDirectoryWalker.java.

[abelsromero]

Another thing to add is to also document and test that files with "." prefix are also not processed.

[abelsromero]

Replaced by #549

[mojavelinux]

Oops. I didn't realize you had filed an issue too. Sorry for not checking first.