Support AGENTS.adoc

[rmannibucau]

Hi markdown is great but it would be neat to support asciidoc format which enables to better format and unify the syntax for a bit more advanced structures (like table).
Technically it shouldn't be crazy for vendors since most of the syntax will overlap enough to get started but it would enable to have more precise context notion and for end users having chosen README.adoc and asciidoc docs an unified format which is a great win for end users.