docs: link to existing documentation for snapshot builds and locally built JARs

[joscha]

Follow-up from #19497 since OpenAPITools/openapi-generator-cli#794 has been merged.

as promised in #19497 (comment)

PR checklist

• Read the contribution guidelines.

• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.

• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

• File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)

• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.