xref validation with github actions

Using XREF Validation with Antora in Github Actions

Documenting conversation on antora gitter for using xref validation in our github actions:

1. You should specify a version tag to use with the validator for stability, DO NOT use the master branch on gitlab:

   Incorrect:

   npm install -g gitlab:antora/xref-validator

   Correct:

   npm insall -g gitlab:antora/xref-validator#v1.0.0-alpha.5

2. Specify the path to site generator modules:

   NODE_PATH="$(npm ls -g --parseable @antora/site-generator-default)/node_modules" antora --generator @antora/xref-validator uyuni-site.yml