Support Asciidoc-Variables for branch names

[bmarwell]

Hi,

you can create status badges like so:

[![Maven CI](https://github.com/bmarwell/cowsay-liberty-feature/actions/workflows/maven.yaml/badge.svg?branch=main)](https://github.com/bmarwell/cowsay-liberty-feature/actions/workflows/maven.yaml)

For asciidoc, a branch name variable seems feasible, but is undocumented. Example: {github.branchname} or similar.

image:https://github.com/bmarwell/cowsay-liberty-feature/actions/workflows/maven.yaml/badge.svg?branch={github.branchname}[link="https://github.com/bmarwell/cowsay-liberty-feature/actions/workflows/maven.yaml",title="Maven CI"]

[gem-cp]

I need the github branchname in asciidoc too. I woukd like to use it for dynamic badges from shields.io to set the right version of an openapi interface in develop, main and feature branches. Right now the badge is only working for one branch.

Example for branch develop: image:https://img.shields.io/badge/dynamic/yaml?url=https%3A%2F%2Fraw.githubusercontent.com%2Fgematik%2Fapi-kim%2Fdevelop%2Fsrc%2Fopenapi%2FServiceInformation.yaml&query=%24.info.version&style=plastic&logo=openapiinitiative&logoColor=green&label=ServiceInformation&color=green[link="src/openapi/ServiceInformation.yaml"]