Bootstrap use of common Gradle docs infrastructure for a native-samples site

[eriwen]

Context

The idea is that we can replace the README.md with an index.adoc with each sample. Asciidoctor extensions and Exemplar would be used for common functionality such as multi-language samples (like those on docs.gradle.org) and testing through Exemplar.

There are a bunch of gaps here:

• It does not generate a separate page per sample. We want it to generate docs/cpp/application/index.html and not docs/index.html.
• Embedded sample tests run with :test, but Exemplar seems to be looking in the wrong place to run samples right now. Also everything is hard-coded for just 1 sample right now.
• The CSS and JS are missing so the generated output doesn't look good yet
• We want to use a CNAME file and DNS such that https://native-samples.gradle.org/cpp/application/ would be the final URL that we'd link to from gradle.org/guides

Contributor Checklist

• Review Contribution Guidelines
• Make sure that all commmits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
• Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by Gradle team
• Ensure that tests pass locally: ./gradlew check