Map maven-site 'locale' to asciidoctor 'lang'

[abelsromero]

What is this issue about?

• Bug report 🪲
• Feature request 🏎
• Question ❓ Consider dropping it first in the Project Chat

Description

Support for captioned titles was added in v3.1.0. We can map the locale set in the maven-site-plugin to lang attribute for better integration.

This should include a section in docs too.

[abelsromero]

Right now it is not possible to know with certainty the locale in use if more than one is present (https://issues.apache.org/jira/browse/DOXIA-755).

But the same as for #969, we can support one locale being set. As a workaround, users can run multiple executions of the plugin each one with a unique locale.